target loads
Yeah, 20 grains would actually be the best load for that for ongoing shooting - just use two lubed felt wads over the powder charge and that should allow your loading ram to fully seat the ball with proper compression. You could use two of the "lube pills" (do a forum search) as well - these are neat little wax paper and hard lube concoctions that you make yourself.
You can also use cream of wheat or ground corn meal as a filler over the charge - you just want to eliminate the air space as mentioned already.
That frame, being a 58 with a top strap is fairly robust even though made of brass, but I would still keep the loads in at the "target" level for most of the shooting.