I would recommend 1 cc of plenum for each FPE you plan for the build.... 1800 psi is pretty low for a .45 cal, are you planning on shooting only roundball?.... Bob
Don't forget, your twist rate is governed by the required stability of your .357 bullet, after the sabot fall away.... Also, by 1:24 do you mean a 24" twist, or 24 x 0.458 = 11".... Bob