as Jethro said, i think that use a $replacex is better
i added the /
since on your original code, you didnt put the sclick for $did 3

on *:dialog:calc:sclick:*:{ 
  if ($did isnum 3-12) {  
    did -a calc 19 $replacex($did,3,/,4,*,5,-,6,7,7,8,8,9,9,+,10,4,11,5,12,6)
  }
  if ($did isnum 14-18) { did -a calc 19 $replacex($did,14,1,15,2,16,3,17,0,18,.) }
  if ($did isnum 22-24) { did -a calc 19 $replacex($did,22,k,23,m,24,b) }
  if ($did == 25) { did -r calc 19 }
  if ($did == 20) {
    if (!$did(19)) { noop $input(Please specify a calculation in the edit box and click calculate,w,Warning!) }
    else {
      did -ra calc 19 $bytes($calc($replace($did(19),k,000,m,000000,b,000000000)),b))
    }
  }
}

this calculator has no accuracy, since if you use 7.3/2 it will return 4