You shouldn't need to do any guessing at all. Install and square the rest, nock an arrow and get it squared within the Berger hole, tighten the rest, install bow square on the string and where the arrow will sit on the rest using the 0 line and then using the vertical measurements on the square by the string install a loose nock in the middle of whatever measurement you need.
However, there is another way to do this (or double check your bow square measurement) without using a square, and provided your bow has an flat cutout shelf.
Nock an arrow and then take another arrow and place it on the shelf and measure in a couple of different locations to make sure you're square. Once you've found your square line, make a reference mark on the string (or use the arrow you've already got nocked as a reference) then use a tape measure to set/check your nock height from there.
The reason why there isn't a measurement along the string, is because that number will likely be different everytime you change strings, adjust poundage, etc... because it will affect ATA, which will in turn change nock and peep heights slightly.
