Identifying shaft taper

[tornitore45]

If you can somehow lock the one inch face of a 1-2-3 block against the shaft you can lay the caliper jaws against each side of the block to obtain measurements spaced exactly one inch apart.

Yap, that is the standard trick used whenever possible, but it still produce errors proportional to the taper angle and the caliper thickness.
I have not tried but sure the geometry has a solution for the taper angle; given the Dmax Dmin, 1" distance and jaw thickness.

[mklotz]

You mount the 1-2-3 block perpendicular to the shaft axis; this eliminates any error due to the taper.

If the thickness of the jaws is 'T' then:

theta = arctan[ 0.5*(d2-d1)/(1+T) ]

yields the half angle of the taper.