My Lyman manual gives the following for 158-grain cast lead round nose in a 4" vented pressure barrel:

HP38, start load 2.6 grains (MV 621, 8800 CUP), max load 4.3 grains (MV 902, 16000 CUP), +P load 4.5 grains (MV 944, 18400 CUP). This should give you a range to work within to achieve your desired results.