I measure the sharpening angle, then double it to get the bevel angle. The sharpening angle is defined by the centerline from the spine through the apex and the line from the spine’s widest point to the apex;

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I just measure the spine width (a) with a caliper, divide that by 2 (a’), then measure the distance between the apex and the widest point on the spine (b) with a machinist’s rule (where it would rest on a sharpening stone). then:

bevel angle = 2 * arcsine(a’/b)