Easiest way to find them is to search in the reviews section for bastard.
Most of the time a rough rule of thumb is best. Calculations and formulae are fine, and sometimes actually reflect reality, but actually trying weights and chronying is the best way. Factors like air density, etc. come into play at times.
Nutshell, as Tys pointed out, ~30fps drop between 0.20g and 0.25g BBs (I'm basing all this on BB Bastards BTW).
I also found there was little if any fps drop when going from 0.28g BBs to 0.30g Bastards, but interestingly, there was a ~20-30fps drop from Bastards to 0.30g SIIS BBs, weight wasn't the difference, Bastards are just bigger and more precise in size/shape.
And most of my testing has shown a ~100fps difference between 0.20g and 0.36g BBs.
So those are the 'rules of thumb' I go with daily.