this piece of code indicates that subtracting 1 from a var works as expected, providing vars are indeed bytes and no overflows occur

Code:
cunder var byte
tempsetc var byte
ct      var byte
ct=50
pause 2000
Debug "Start",13 ,10    
     
    for   tempsetc = 1   to 255
        cunder = (tempsetc - 1)  
        Debug 13 ,10,"cunder = " ,#tempsetc," - 1 = ", #cunder 
        if ct <= cunder then
            Debug 9, #ct ," <= ", #cunder
        else
            Debug 9, #ct ," > ", #cunder
        endif
        next    
 stop
result

Code:
Start
cunder = 1 - 1 = 0 50 > 0
cunder = 2 - 1 = 1 50 > 1
cunder = 3 - 1 = 2 50 > 2
cunder = 4 - 1 = 3 50 > 3
cunder = 5 - 1 = 4 50 > 4
cunder = 6 - 1 = 5 50 > 5
cunder = 7 - 1 = 6 50 > 6
cunder = 8 - 1 = 7 50 > 7
cunder = 9 - 1 = 8 50 > 8
cunder = 10 - 1 = 9 50 > 9
cunder = 11 - 1 = 10 50 > 10
cunder = 12 - 1 = 11 50 > 11
cunder = 13 - 1 = 12 50 > 12
cunder = 14 - 1 = 13 50 > 13
cunder = 15 - 1 = 14 50 > 14
cunder = 16 - 1 = 15 50 > 15
cunder = 17 - 1 = 16 50 > 16
cunder = 18 - 1 = 17 50 > 17
cunder = 19 - 1 = 18 50 > 18
cunder = 20 - 1 = 19 50 > 19
cunder = 21 - 1 = 20 50 > 20
cunder = 22 - 1 = 21 50 > 21
cunder = 23 - 1 = 22 50 > 22
cunder = 24 - 1 = 23 50 > 23
cunder = 25 - 1 = 24 50 > 24
cunder = 26 - 1 = 25 50 > 25
cunder = 27 - 1 = 26 50 > 26
cunder = 28 - 1 = 27 50 > 27
cunder = 29 - 1 = 28 50 > 28
cunder = 30 - 1 = 29 50 > 29
cunder = 31 - 1 = 30 50 > 30
cunder = 32 - 1 = 31 50 > 31
cunder = 33 - 1 = 32 50 > 32
cunder = 34 - 1 = 33 50 > 33
cunder = 35 - 1 = 34 50 > 34
cunder = 36 - 1 = 35 50 > 35
cunder = 37 - 1 = 36 50 > 36
cunder = 38 - 1 = 37 50 > 37
cunder = 39 - 1 = 38 50 > 38
cunder = 40 - 1 = 39 50 > 39
cunder = 41 - 1 = 40 50 > 40
cunder = 42 - 1 = 41 50 > 41
cunder = 43 - 1 = 42 50 > 42
cunder = 44 - 1 = 43 50 > 43
cunder = 45 - 1 = 44 50 > 44
cunder = 46 - 1 = 45 50 > 45
cunder = 47 - 1 = 46 50 > 46
cunder = 48 - 1 = 47 50 > 47
cunder = 49 - 1 = 48 50 > 48
cunder = 50 - 1 = 49 50 > 49
cunder = 51 - 1 = 50 50 <= 50
cunder = 52 - 1 = 51 50 <= 51
cunder = 53 - 1 = 52 50 <= 52
cunder = 54 - 1 = 53 50 <= 53
cunder = 55 - 1 = 54 50 <= 54
cunder = 56 - 1 = 55 50 <= 55
cunder = 57 - 1 = 56 50 <= 56
cunder = 58 - 1 = 57 50 <= 57
cunder = 59 - 1 = 58 50 <= 58
cunder = 60 - 1 = 59 50 <= 59
cunder = 61 - 1 = 60 50 <= 60
cunder = 62 - 1 = 61 50 <= 61
cunder = 63 - 1 = 62 50 <= 62
cunder = 64 - 1 = 63 50 <= 63
cunder = 65 - 1 = 64 50 <= 64
cunder = 66 - 1 = 65 50 <= 65
cunder = 67 - 1 = 66 50 <= 66
cunder = 68 - 1 = 67 50 <= 67
cunder = 69 - 1 = 68 50 <= 68
cunder = 70 - 1 = 69 50 <= 69
cunder = 71 - 1 = 70 50 <= 70
cunder = 72 - 1 = 71 50 <= 71
cunder = 73 - 1 = 72 50 <= 72
cunder = 74 - 1 = 73 50 <= 73
cunder = 75 - 1 = 74 50 <= 74
cunder = 76 - 1 = 75 50 <= 75
cunder = 77 - 1 = 76 50 <= 76
cunder = 78 - 1 = 77 50 <= 77
cunder = 79 - 1 = 78 50 <= 78
cunder = 80 - 1 = 79 50 <= 79
cunder = 81 - 1 = 80 50 <= 80
cunder = 82 - 1 = 81 50 <= 81
cunder = 83 - 1 = 82 50 <= 82
cunder = 84 - 1 = 83 50 <= 83
cunder = 85 - 1 = 84 50 <= 84
cunder = 86 - 1 = 85 50 <= 85
cunder = 87 - 1 = 86 50 <= 86
cunder = 88 - 1 = 87 50 <= 87
cunder = 89 - 1 = 88 50 <= 88
cunder = 90 - 1 = 89 50 <= 89
cunder = 91 - 1 = 90 50 <= 90
cunder = 92 - 1 = 91 50 <= 91
cunder = 93 - 1 = 92 50 <= 92
cunder = 94 - 1 = 93 50 <= 93
cunder = 95 - 1 = 94 50 <= 94
cunder = 96 - 1 = 95 50 <= 95
cunder = 97 - 1 = 96 50 <= 96
cunder = 98 - 1 = 97 50 <= 97
cunder = 99 - 1 = 98 50 <= 98
cunder = 100 - 1 = 99 50 <= 99
cunder = 101 - 1 = 100 50 <= 100
cunder = 102 - 1 = 101 50 <= 101
cunder = 103 - 1 = 102 50 <= 102
cunder = 104 - 1 = 103 50 <= 103
cunder = 105 - 1 = 104 50 <= 104
cunder = 106 - 1 = 105 50 <= 105
cunder = 107 - 1 = 106 50 <= 106
cunder = 108 - 1 = 107 50 <= 107
cunder = 109 - 1 = 108 50 <= 108
cunder = 110 - 1 = 109 50 <= 109
cunder = 111 - 1 = 110 50 <= 110
cunder = 112 - 1 = 111 50 <= 111
cunder = 113 - 1 = 112 50 <= 112
cunder = 114 - 1 = 113 50 <= 113
cunder = 115 - 1 = 114 50 <= 114
cunder = 116 - 1 = 115 50 <= 115
cunder = 117 - 1 = 116 50 <= 116
cunder = 118 - 1 = 117 50 <= 117
cunder = 119 - 1 = 118 50 <= 118
cunder = 120 - 1 = 119 50 <= 119
cunder = 121 - 1 = 120 50 <= 120
cunder = 122 - 1 = 121 50 <= 121
cunder = 123 - 1 = 122 50 <= 122
cunder = 124 - 1 = 123 50 <= 123
cunder = 125 - 1 = 124 50 <= 124
cunder = 126 - 1 = 125 50 <= 125
cunder = 127 - 1 = 126 50 <= 126
cunder = 128 - 1 = 127 50 <= 127
cunder = 129 - 1 = 128 50 <= 128
cunder = 130 - 1 = 129 50 <= 129
cunder = 131 - 1 = 130 50 <= 130
cunder = 132 - 1 = 131 50 <= 131
cunder = 133 - 1 = 132 50 <= 132
cunder = 134 - 1 = 133 50 <= 133
cunder = 135 - 1 = 134 50 <= 134
cunder = 136 - 1 = 135 50 <= 135
cunder = 137 - 1 = 136 50 <= 136
cunder = 138 - 1 = 137 50 <= 137
cunder = 139 - 1 = 138 50 <= 138
cunder = 140 - 1 = 139 50 <= 139
cunder = 141 - 1 = 140 50 <= 140
cunder = 142 - 1 = 141 50 <= 141
cunder = 143 - 1 = 142 50 <= 142
cunder = 144 - 1 = 143 50 <= 143
cunder = 145 - 1 = 144 50 <= 144
cunder = 146 - 1 = 145 50 <= 145
cunder = 147 - 1 = 146 50 <= 146
cunder = 148 - 1 = 147 50 <= 147
cunder = 149 - 1 = 148 50 <= 148
cunder = 150 - 1 = 149 50 <= 149
cunder = 151 - 1 = 150 50 <= 150
cunder = 152 - 1 = 151 50 <= 151
cunder = 153 - 1 = 152 50 <= 152
cunder = 154 - 1 = 153 50 <= 153
cunder = 155 - 1 = 154 50 <= 154
cunder = 156 - 1 = 155 50 <= 155
cunder = 157 - 1 = 156 50 <= 156
cunder = 158 - 1 = 157 50 <= 157
cunder = 159 - 1 = 158 50 <= 158
cunder = 160 - 1 = 159 50 <= 159
cunder = 161 - 1 = 160 50 <= 160
cunder = 162 - 1 = 161 50 <= 161
cunder = 163 - 1 = 162 50 <= 162
cunder = 164 - 1 = 163 50 <= 163
cunder = 165 - 1 = 164 50 <= 164
cunder = 166 - 1 = 165 50 <= 165
cunder = 167 - 1 = 166 50 <= 166
cunder = 168 - 1 = 167 50 <= 167
cunder = 169 - 1 = 168 50 <= 168
cunder = 170 - 1 = 169 50 <= 169
cunder = 171 - 1 = 170 50 <= 170
cunder = 172 - 1 = 171 50 <= 171
cunder = 173 - 1 = 172 50 <= 172
cunder = 174 - 1 = 173 50 <= 173
cunder = 175 - 1 = 174 50 <= 174
cunder = 176 - 1 = 175 50 <= 175
cunder = 177 - 1 = 176 50 <= 176
cunder = 178 - 1 = 177 50 <= 177
cunder = 179 - 1 = 178 50 <= 178
cunder = 180 - 1 = 179 50 <= 179
cunder = 181 - 1 = 180 50 <= 180
cunder = 182 - 1 = 181 50 <= 181
cunder = 183 - 1 = 182 50 <= 182
cunder = 184 - 1 = 183 50 <= 183
cunder = 185 - 1 = 184 50 <= 184
cunder = 186 - 1 = 185 50 <= 185
cunder = 187 - 1 = 186 50 <= 186
cunder = 188 - 1 = 187 50 <= 187
cunder = 189 - 1 = 188 50 <= 188
cunder = 190 - 1 = 189 50 <= 189
cunder = 191 - 1 = 190 50 <= 190
cunder = 192 - 1 = 191 50 <= 191
cunder = 193 - 1 = 192 50 <= 192
cunder = 194 - 1 = 193 50 <= 193
cunder = 195 - 1 = 194 50 <= 194
cunder = 196 - 1 = 195 50 <= 195
cunder = 197 - 1 = 196 50 <= 196
cunder = 198 - 1 = 197 50 <= 197
cunder = 199 - 1 = 198 50 <= 198
cunder = 200 - 1 = 199 50 <= 199
cunder = 201 - 1 = 200 50 <= 200
cunder = 202 - 1 = 201 50 <= 201
cunder = 203 - 1 = 202 50 <= 202
cunder = 204 - 1 = 203 50 <= 203
cunder = 205 - 1 = 204 50 <= 204
cunder = 206 - 1 = 205 50 <= 205
cunder = 207 - 1 = 206 50 <= 206
cunder = 208 - 1 = 207 50 <= 207
cunder = 209 - 1 = 208 50 <= 208
cunder = 210 - 1 = 209 50 <= 209
cunder = 211 - 1 = 210 50 <= 210
cunder = 212 - 1 = 211 50 <= 211
cunder = 213 - 1 = 212 50 <= 212
cunder = 214 - 1 = 213 50 <= 213
cunder = 215 - 1 = 214 50 <= 214
cunder = 216 - 1 = 215 50 <= 215
cunder = 217 - 1 = 216 50 <= 216
cunder = 218 - 1 = 217 50 <= 217
cunder = 219 - 1 = 218 50 <= 218
cunder = 220 - 1 = 219 50 <= 219
cunder = 221 - 1 = 220 50 <= 220
cunder = 222 - 1 = 221 50 <= 221
cunder = 223 - 1 = 222 50 <= 222
cunder = 224 - 1 = 223 50 <= 223
cunder = 225 - 1 = 224 50 <= 224
cunder = 226 - 1 = 225 50 <= 225
cunder = 227 - 1 = 226 50 <= 226
cunder = 228 - 1 = 227 50 <= 227
cunder = 229 - 1 = 228 50 <= 228
cunder = 230 - 1 = 229 50 <= 229
cunder = 231 - 1 = 230 50 <= 230
cunder = 232 - 1 = 231 50 <= 231
cunder = 233 - 1 = 232 50 <= 232
cunder = 234 - 1 = 233 50 <= 233
cunder = 235 - 1 = 234 50 <= 234
cunder = 236 - 1 = 235 50 <= 235
cunder = 237 - 1 = 236 50 <= 236
cunder = 238 - 1 = 237 50 <= 237
cunder = 239 - 1 = 238 50 <= 238
cunder = 240 - 1 = 239 50 <= 239
cunder = 241 - 1 = 240 50 <= 240
cunder = 242 - 1 = 241 50 <= 241
cunder = 243 - 1 = 242 50 <= 242
cunder = 244 - 1 = 243 50 <= 243
cunder = 245 - 1 = 244 50 <= 244
cunder = 246 - 1 = 245 50 <= 245
cunder = 247 - 1 = 246 50 <= 246
cunder = 248 - 1 = 247 50 <= 247
cunder = 249 - 1 = 248 50 <= 248
cunder = 250 - 1 = 249 50 <= 249
cunder = 251 - 1 = 250 50 <= 250
cunder = 252 - 1 = 251 50 <= 251
cunder = 253 - 1 = 252 50 <= 252
cunder = 254 - 1 = 253 50 <= 253
cunder = 255 - 1 = 254 50 <= 254

the problem lies elsewhere

you need to provide a compliable example that demonstrates the problem