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Brad's razors in this thread are all angled between 7.7 and 9.7 degrees when lying flat on the face. So at least for those razors, one to two spiine widths off the face would be no less than 15.4...

Absolutely right!

I still haven't achieved the dexterity required to safely hold and read a protractor in one hand while shaving with a sharp object in the other. Practice is the best method
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When I shave with a wedge against the grain, I keep the blade flat on my face. I don't know why it does, but it works well for me. Except when the suction between my skin and blade traps the razor...