I did not check every step, but it certainly seems you are on track - the math seems simple enough when broken into steps and, presuming you follow syntax rules, it should work fine. A few notes:

As you are working with byte values, it will likely be easiest to manipulate maidenhead in an array - each character in its own element as: MH[0] .. MH[5]. In this way you may loop

EL = 0 to 5
MH[EL] 'Do something with MH[EL] - serial out...
Next EL

If you want to "Put them together in pairs", you can assign them to a word variable with .HIGHBYTE, .LOWBYTE. Perhaps this will work well, if Highbyte is always a letter (or rather the ASCII value associated) and Lowbyte is always a number. Take a look at ARRAYWRITE and ARRAYREAD also.

Couple of other notes:
Capital letters are related (value wise) to lowercase letters, one LOOKUP will do- Check an ASCII table, but I think adding 40 is lowercase.
You may assign results to a variable in the formula as MH[0] = MH[0] + 5. In this way (sometimes) the code is clearer.

All I got - nothing earthshaking, just some thoughts as I consider how I might do it. Oh, almost forgot... Good luck!