The older Speer Manuals (and maybe the new ones) have a section on this with the Speer shot cups. They listed 4.5 grains of Bullseye in the 38's, but also mentioned that lower power = better patterns. That has been my experience and I stick with 3 gr of Bullseye.
The estimated pattern spread was 1" per foot of distance. (20 feet gives a 20 inch spread) These aren't a long range proposition. jd