There is no single correct answer to your question. As pointed out in the previous posts, primers are not all you are compensating for. Also, you may have noticed that even modern shotgun shell load recipes are primer-specific. In other words, even modern primers are not all exactly the same as compared to each other, much less as compared to the old #57, so compensation is likely a bit different for each one.
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The only two really safe ways I know to do what you are trying to do are to send a series of your loads for professional pressure testing or to buy a
Pressure Trace and measure the pressures from several commercial loads and then start upping your load until you get a peak pressure match. Both of those options are expensive.
A much less reliable option, but one that can be used to get into the ballpark, if done with some care, is to use a chronograph to measure your muzzle velocity. This can be tricky with a shotgun. SAAMI lists three methods, optical chronograph at 15 feet for lead shot, optical at 8 feet for steel shot, and, for either, inductance sensors at 3 feet. The inductance sensor velocity numbers are the ones listed by SAAMI in
the SAAMI standard. I suspect the Magnetospeed V3 type chronograph may be the most viable amateur option, but if you don’t already have one, the lowest price I found was $400. Unfortunately, the less expensive model is “for centerfire rifles only.”. The way you would use it is to look at the manufacturer's published velocities for some commercial loads and compare your measurements to that claim. If, due to barrel length or diameter or choke or chronograph differences, they came out measuring some percent slower than claimed on the box, then I would assume you need to adjust all your expectations for published load performance by that same percentage. We already know your velocities with the paper hull and fiber wads are likely slower than plastic component loads, so look at the velocity you actually get for your current reloads and gradually increase the charge to get to the claimed load velocity that has been adjusted by the percentage for your barrel.
In theory, as long as your gun needs MORE powder to get to your adjusted published load data velocity than the manual says will achieve that velocity, your peak pressure should actually be a little lower than the original load produced. But any time LESS powder is getting you to the published velocity as adjusted for your gun, your peak pressure is higher, and it is not recommended.
Sorry it’s so complicated, but that’s sort of where it goes with interior ballistics.