The general FPS rules are based on power levels at the muzzle.
They are translated into mere-mortal terms (i.e. simple for dummies like me) into:
Outdoors: 400fps with 0.20g BBs
Indoors: 350fps with 0.20g BBs
If you shoot a heavier BB the FPS might be lower, but that's not the point. If you shoot a heavier BB at the same 400fps...you're obviously shooting much "harder".
There are NO exact translation numbers such as..."if you shoot XXXfps with 0.20g BBs...you'll shoot YYYfps with a different weight BB". That's why the standard is 0.20g BBs....it's just everyone leaves it out of the rules when they post.
Also note...for indoor. There's typically no wind and shorter distances involved. "Light" 0.20g BBs are 99% sufficient for indoors. "heavier" BBs just hurt more....and that's not really the point.