You've got me. I don't think there is ever a rule of thumb when it comes to determining actual pressures. In regard to black powder, I am baffled by the fact that there are literally thousands of smokeless loads published with pressure information, yet I have never seen the same information published for black powder. Maybe I'm just not looking in the right places. Sherman Bell has published comparitive info in the DGJ, but, I believe, only for 12 ga. and possibly 10 ga. I would like to see Sherman Bell or Tom Armbrust pressure test a few loads in 10, 12, 16, and 20 ga with black powder and publish the results, as I think there is quite a bit of interest. However, from what I can determine, they are already very busy guys. Maybe one reason these tests have not been done or published (I assume) is that there is a general feeling that appropriate black powder loads in any gun in good condition are safe, because that is what the older guns were designed to shoot. Hopefully, someone more informed than myself can shed some light on the subject.