I use this formula:

Subtract 10 fpsfor every 1" you are under IBO length.
Subtract 1 fps for every three grains you are over IBO.
Subtract 2 fps for every pound you are from IBO.
Subtract5 fps for a peep site.
Subtract5 fps for a kisser button.
Subtract 4 fps for a string loop.

I used this formula and was off by 1 fps.