The formula is different for everyone to get final twist ratio as everyone twist different wether tighter or looser but what I would do is let’s say your gonna make a string for a 62” AMO longbow which would typically be a final string length of 59” now lay out your string at what you think is a 62” string setting now get your bundles together measure down 9” and start twisting and make your desired loop size and after you fold over your loop and marry the bundles together you keep twisting and twist 3” past your finished strands and now tie that with a bread tie so now measure from end of loop to where you finished that should be bundle ratio of let’s say 7.5” now do your back twist and measure again and start at 9” and make your next loop and repeat what you did on the first loop and now add your twist to the bundle (20 twist min) pull tight and stretch so what ever that string length is adjust your peg to that string size
I personally like my string ratio to be at least 2 twist per inch or 1 twist every 3/4”
You can fine tune at this point to get to YOUR ratio
Now this is one method to get your feet wet. I have other methods and I personally like my pegs marked for actual string length and not AMO because not all bowyers follow the old rule of thumb because they can be plus or minus 1” to 1.5”
Hope this helps and if more info needed give a shout