Steve, it's going to be around 4468 +/- 10% fps BUT will also be high pressure at around 857 bar (taken on a COAL of 2.680"). 56.8grains will give a compressed charge of 35% are you sure your numbers are correct?You can not get 100% accurate MV unless you spend £1000's on a top notch chronograph. The cheaper ones have an error of +/- 5% IIRC. You can obtain a mathematical model of your load using "Quick load" which is surprisingly very accurate.
John
Oops, sorry all, not 56.8 it should have been 46.8gr.