Your idea of a powder charge sounds reasonable to me -- does the bullet have
any manner of a crimp groove? I should think that it would. I'd simply aim for that crimp groove.
Given that you are talking about a lead bullet that is slightly under-weight for the caliber, and you are talking about .38 Special launched from a .357 Magnum revolver, I can't see you getting yourself in any kind of trouble.
Wish I had some genuine help for you.