Calvin, people have generally said here .025 for a 12ga , but the British proofing houses will pass down to .020.
I am personally uneasy with anything close to .025 in a 12. Many light small bores came from the factory at close to .025, so that is a bit different. Most Parker 12's were factory produced at least .035. I would have to look up the hoop stress formulae again, but the greater the diameter, the greater the wall thickness needs to be to withstand the same internal pressure.
I know you are a 10ga guy and I don't have enough experience with the 10's to know the average factory wall thickness on them or what would be acceptable after honing. I know lots of 10 ga guys shoot some heavy loads. If your intent is to load 1 1/4 3dre, then that is such a light 10 load, it wouldn't make much difference. If you want to experiment and see how much you can get, you need to know more than I can tell you.
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