I went with gut feel & tried some 18.5 Gr loads & visually they seem to be about 85~95% capacity with the bullet seated to the cannelure to crimp. The load was pretty sooty so I'm guessing low(ish) pressure. I went up to 19.0 Gr & the dirt cleaned up quite a lot so I loaded a small batch to chronograph. Any more volume & I think you're going to HAVE to compress a ball powder, something I'm just not comfortable with.
How much the powder fills the case is not a good indicator of anything.
There is no special issue with compressing ball powders.