Jim wrote:

The taper angle is .7768 degrees. Just the the Tangent Opposite Adjacent trig method.

I'm perfectly capable of figuring that out, it just is the hard way to
do it...what you gonna' use to set 0.78 degrees? Meanwhile you can
measure 1/2" pretty darn close pretty quick. Or, since OP added that
he's making the neckpiece for a git-tar, he'll want to taper it half on
each side as someone else noted. That would be 0.39 degree, even
tougher to set w/ anything...

IMO, YMMV, $0.02, etc....