It really doesn't matter what numbers are marked on your board; just think of them as somewhat arbitrary index marks. On my board, the way the string doubles back on itself, every time I move a peg back 1", it increases the overall string length 4". Figure this out for your board, which may be different from mine. Knowing this, you should be pretty close on your second attempt. For example, if I make a string that is 2" too short, on my next attempt I will move my pegs back 1/2". Of course, as you're learning to do this, you're not going to be totally consistent from string to string, so whatever adjustment you make might not be perfect.
Then when you get a string that is right, mark down everything on a card, and you'll know how to make a perfect string for that bow the next time. Mark down type of string material, number of strands, pigtail lengths (top and bottom nocks), nock loop sizes, which pegs used, and number and types of nock padding strands (if used). I get pretty precise on this. If I feel Iike my string is okay, but has a few too many twists, I'll write down a peg that is 1/4" shorter than the one I actually used, thus decreasing the string length by 1", so that the next string I make will end up having fewer twists.
I have enough cards now that I can usually make a string that works the first time for any bow I'm likely to make one for, even if the bow is a different length than I've made one for before, by interpolating from one or more of my cards.