Depends on what Lyman bullet ya got the data from. If it's from the #356311 round-nose or #358429 Keith-style bullets , they have the weight in the long nose , and leave more room in the case. Other SWCs will leave less room in the case , and possibly a compressed charge.

Looking thru a few tables , I see all kind of max weights for W-231/HP-38 and a 158gr lead bullet. From 3.7 to 4.9.

Pressures and recommended charges can vary due to a number of factors. Powder lot to lot variations , different test methods and equipment , atmospheric conditions , primer used , hardness of cast bullets vs swaged lead , etc.