I just tried this calculator. Finally, something that is reasonably close to what three chronographs say. It calculated about 4 fps faster than my bow actually shoots. The bow is a Martin Slayer/Nitrous B cams, 27" draw, 53# and a 267gr arrow (3D). The only thing on the string is a Fletcher True peep, a tied-on nockset and 3" center serving. The bow chrono's at 299 fps and the calculator gave it 303 fps. Fairly close.

For those wanting some detail it asks for the weight of stuff on the string. FYI that includes the center serving