Here's how I did my horn nocks if it helps, I got a 12mm hole cutting drill bit and ground it to a spear leaf shape, I also use it as a template to shape the limb tip to get a snug fit. The width at the socket ends up at 11mm - 12mm and the depth of the drill hole I go to is 45mm in a length of 2" horn. Once glued on I then shape the horn nock, mostly with a horse rasp.
R.D.