The safest way is finding with an equation and testing with a chrony. It is a mathematic formula with a given number of Joules that will give you the FPS in relation with the weight of the BB. You use that number in relation with the chrony and you are good to go.
My chrony can test both 0.2 and 0.25. So I know that for CQB in aiming for 314 FPS with .25 that's it. The same number of joule 1.14 joule than a 0.2 BB going at 350 FPS.