I worked up to 4.5 grains of 231 under a 158 LSWC in .38, starting from 4.0 grains.

4.0 was noticeably dirty, 4.2 was still dirty, 4.4 was definitely better but 4.5 got the velocity I wanted from my 3" M10; it clocks at 804 fps average for 12 shots, making a 125 power factor.

My 4" M15 routinely gets 75 fps faster velocities, but I haven't checked this particular load.