I feel your pain, it does require some patience and practice.
I start with the tillering nocks as small as I can get 'em to allow for adjustment.
As some have suggested, a jig may help...
or here's an idea, once you have a shallow groove on one side, tape a cocktail stick or sliver of wood into the groove, so that when you turn the bow over the line of the nock is still visible.
Any excuse for a Martini
I tend to end up with somewhat personal style of nock, probably as a carry over from my crossbow making days. Some of those fancy longbow horn nocks get a bit over the top, like darned great chess men stuck on the end of the bow.
Del