I never saw the slightest use in these string jigs. I just measure the tillering string and add about 35 to 40cm when I cut the FF, dacron or whatever string material. I must add, that I splice the second loop in, that makes it easier for me to judge where it has to be placed. Plus I have some freedom to play with the total length of the string.
But that's just my way...
Regards,
gian-luca