the purpose is not to chorno velocity.
It's to chrono MUZZLE ENERGY.
a .2 at 400 fps for example is 1.48 Joules.
a .3 at 327 fps is 1.48 Joules.
The problem with getitng people to chrono with the weight they are shooting is that unless you have a scale at the chrono station, you can't prove that the person claiming to have .3s in his gun are truly .3s.
It's easier to provide the ammo as a game organizer at the chrono station and ensure everyone is shooting 1.48J with .2s.
Well equipped game organizers could provide a heavier weight to cross reference the J output of a gun, especially if players intend to use heavier ammo... because you can start to see Joule creep in some setup. you can do a search for that, I'm not explaining that here. The TLDR is that the velocity/muzzle energy at the chrono is higher than the math suggests if you were to convert your .2 velocity/output to .3 output instead of measuring it.
As for YOUR attempt at interpreting it.
A .2 at 490fps is 2.22J
If that same gun were to chrono with a .3 mathematically it should be chronoing at 400fps to be at 2.22J
2 different velocities, 2 different weights, same muzzle energy.
I futz with V2s, V3s and V6s. I could be wrong... but probably, most likely not, as far as I know.