There are at least two good ways to figure out the right string length. Both assume a string with a loop on only one end - the other end tied with a bowyer's knot.
1) Use your tillering string to get the fistmele you want, then measure between the notches. Add on length for the loops - be sure to make it slightly long. You can make final adjustments with string twist.
2) I build my strings with only one loop, the length of the bow, which leaves plenty of length for the single Flemish loop. Use a bowyer's knot on the lower end and make the course adjustment there - leaving it slightly long. I then remove the loop end from the bow and twist for fine tuning. It only takes a few minutes to get it exact.
Bill