for 10 bits ...

use V ref = 4.096v or 2.048 v ... but not 2.5 v

for 4.096 v you get 1 count for 1 mv
for 2.048 v you get 2 counts for 1 mv input ...

so, your formula ... is really awful !!!

100°C = 1 v ...

with a 2.5 v ref : 1v /2.5 *1024 counts ... 409.6 > 409

409*2500/256 / 4 = 998.5 ...

you will find the good numbers easily !

Alain