Well according to your first sentence of your org post SPJ, you're asking for response from someone with experience in loading .357 cases with black powder. I have none, but alot with reloading 38 Special, 44 Colt, 45 Colt, and some with 44 Special. In regard to the 38 Spec, I reload with 18.5 grains of FFF using a Snakebite cast lead bullet (Dick Dastardly) that drops at around 160 grains. Good accuracy out of my Uberti Richard/Mason model conversion and Marlin 1894 Cowboy model rifle. No excessive blast, burns well, and normal residue.
A .357 case is approximately 1/8" longer, not sure what the max or best load would be, but as with most metal cased black powder loadings, the rule of thumb is to drop enough powder to where the bottom of a seated bullet would reach, then add enough to get good compression. Nothing excessive, just enough to compress. As with all black powder reloading you don't want any airspace in the round nor excessive compression so trying to play around with less or more with black powder loadings doesn't leave alot for experimentation. Not like playing around in reloading minimum/maximum powder charges as with smokeless powders. A person can experiment using either FF or FFF to see what works best. Have read that anything below 44 calibers use FFF, anything above best results can be had with FF.
I reload alot of 44 Colt with FF black using a Mav Dutchman 200 grain cast lead bullet. Most of the time I use 25.5 grains. I have a few times loaded up some 44 Special rounds with the same bullet using 28.5 grains of FF black. They shot all right but didn't perform any better than the 44 Colt loadings in my Uberti Richards II, Open Top, and Model 66 Yellow Boy rifle, saw no need to use 44 Spec black in these firearms so just use 44 Colts. The 44 Special case is around 1/16" longer than the 44 Colt. Being the .357 mag case is 1/8" longer than the 38 Spec it will of course hold more, the 25.0 grains of FFF you mentioned SPJ might be the ticket, but I'd do the seated bullet height of powder in the case plus a bit more to get good compression. You didn't mention if your shooting goals for accuracy is for Cowboy action shooting, target, or hunting. Just curious.