you dont need both of these

elseif ($did(3) isletter) { did -ra tmp 5 Please use numbers only. | halt }
    elseif ($did(3) !isnum) { did -ra Tmp 5 Please enter a number. | halt }

just do

elseif ($did(3) !isnum) { did -ra tmp 5 Please use numbers only. }

and maybe have it disable the convert button till you enter a number in the edit box
but all in all it\'s a nice lil dialog