There's detailed instructions for a string board that goes by string length in "Doin' the Twist" and "How to Build a Great Bowstring", but you can also go with the advice given and re-mark the board you have.
Do a little measureing. For instance, wind off a strand on a given peg--say 60" AMO on your board (should make a 57" string). Figure out how much you use for your loops (usually around 8"). Double that, and subtract from the length of the strand--that should be the length of string that peg will make. The pegs should be marked in 1" increments--re-mark accordingly, AFTER making a string to verify you have the first peg re-marked correctly.
Never understood why jigs were marked by bow length...string length can vary a bunch depending on the bow. One 60" bow might get a 54" string, anothter 56", another 57", another 58"...lots easier to mark it by string length.
Chad