Well this is one calculator that was at least in the ball park for my setup. My bow chrono's at 275 fps and the calculator says 280 fps. The only variable is that I use a Specialty SuperBall Peep with a Verifier lens and estimated it at 20 grains. It might be a little heavier than that, Like I said, at least it's somewhat close. Some of these on-line calculators give results of 286 fps. I could only wish........