This is the formula for damage. So you would fill in the spaces that you know, and after that it is simple math to find out the base power. Or in your specific case you could get your Pokemon to level 50 or 100 then use an IV calculator to find out the IVs and Hidden power. Or if you are in Gen IV then you can do a Lvl. 100 Single instead of leveling up.
EDIT: As DT mentions in the comments, part of the modifier is a random number from 0.85 to 1.0. So this won't be totally reliable, but you can run it a few times and take the average.