It probably depends on the length of your bow, and possibly, if you are shooting with a rest of off the shelf. I used the Tuning for 10's approach to set my nock height. I was surprised when it came out as 13/16th. I did some research and found that 3/4 is pretty standard for 3 under. I got this from John Magera which I verified doing some on line searches. John is a traditional archer who crossed over and made the Olympic team a few years back. Maybe that 3/4 is for longer FITA style barebows. It is measured from the line that is at rest, or shelf level. The diameter of the arrow is include in the number so the center line of a fat arrow will be lower than the center line of a skinny arrow. My 13/16 is with skinny Victory VAP.