Theoretically it would make sense to have the nock point exactly in line with the arrow rest at 90 degrees from the string...
But that's only if you have a softish arrow rest.
Us hardy English fellows
shoot our longbows off the knuckle, which can get gouged by the quill of the lower fletching as it leaves the bow.
The cure for this is to raise the nocking point just enough for the back end of the arrow to clear the hand as it leaves the bow.
Raise it too much and it will porpoise.
Having the nocking point too low is also likely to cause porpoising.
Like most of this stuff I think experimentation is the best way to find out. also taking it to extremes may also illustrate what happens. so if you nock 2 inches low and then let the bow down slowly you will see what the arrow has to do to get away.
Del