uhmm, please verify the operating pressure on that certificate.
I was prepared to scold you for risking life and limb by contemplating the use of Pyro RS in such a low operating pressure weapon. So I drug out my copy of Lyman's "Blackpowder Handbook & Loading Manual" 2nd edition written by Sam Fadala, flipped back to .58 cal rifle section, and found something very different than what I expected. In my .54 cal's Goex FF gives lower pressures than Pyro RS with equal bullet weights. In the .58 cal's that are listed in this manual Pyro RS gives a much lower pressure. Loading .577 dia 566 grain minnie ball projectiles, 80 grains of Goex FF gives a pressure of 8,900 psi, and yields a velocity of 1,058 fps. Same projectile, 100 grains load of Pyro RS gives a pressure of 9,100 psi with a velocity of 1,169.
Load it like you want to and have a blast!! (After you verify my findings, as well as the certificate info.)