If I put brackets around the dividing part will that fix it?
That'll truncate the division, leaving you with 412/255 = 1

You can use PBPL mode so that it uses LONGs, which should work but it'll be larger and slower if that matters. Otherwise your best bet might be to turn the equation into a lookup table.
To be honest, I don't use PBP much, so maybe others can suggest something else.