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Or you could just keep above the 1 / 4 ratio and don't bother with formulae.
That is what vintage makers did and it seems to work well.

Yes, that is correct. There is a variance of course. Not all razors are exactly the same in angle. But 15 to 16 seems right.

The correct way to measure, for the purpose of geometry, is from the edge to the point where the spine makes contact with the hone. At that point, the thickness vs width (edge to spine contact)...