A nock point of 3/4 to 7/8 would be normal for a 3 under shooter. In fact, if you nock lower than that you can get a false high, where the arrow is bouncing off the shelf and showing nock high whereas it is really nock low. The only way you can determine where your nock point should be is to initially nock high enough that you are sure you are getting a true nock high, and work your way down from there.
A nock point of 7/8 would be a little high for a split finger shooter. In that case, I would look for form problems. In order to get good arrow flight, whether dealing with weak/stiff shaft issues, or porpoising issues, you need to have solid form, where you are coming to full draw (meaning that your drawing forearm is lined up with the arrow viewed from above), and continuing to pull with good back tension until the arrow is released (meaning that your drawing hand moves backwards on release, and not to the side). If you are shooting split fingers, and your drawing elbow is too high, viewed from the side, it can cause your index finger to press down on the arrow, which will cause it to bounce on release and you won't get good arrow flight.