After you find what the limit on your rifle is then you have to remember all powders are not of the same potential energy. Most limits are based on black powder by volume, and you have to understand the pressure relationship because FFF with burn faster andgenerate more pressure for the same amount of powder than FF for the same amount of powder but the potential energy is the same.
Remembering that I am stating this as my opinion, I will try to give you a kind of comparison.
Black powder FF pressure about the same per volume as RS Pyrodex so I use the same volume.
Black FFF is a bitweaker and generates a bitless pressure than P Pyrodex.
150 gr of black generates about the same pressure as 115 gr of 777
150 gr of black generates about the same pressure as 120 gr of Blackhorn 209.
Now remember two things about what I am saying here , I use GOEX black other brands may differ and I never mentioned velocity the relationship between velocity and pressure is NOT direct in the fact that how long the pressure is maintained is what controls velocity.
I hope this helps. Lee