make a tillering string a bit longer than necessary and it will do for all future bows, you can make a handy 'string shortener' to adjust it as you are tillering and then use that length to make the final string.
The string shortener is just an off cut of good hard wood or ply about an inch or so long and half an inch wide (about 3/16" thick) with two holes drilled through it with about a 1/4" between the two holes , a bit like a figure
8 smoothe the edges of the holes and thread the stringin one hole and back through the other, repeat the threading through and back to shorten the string. It will save a lot of messing about The 'Del the Cat String Shortener patent # 83467794231'
This pic shows it in use on the string (on the right side of the draw point)
BTW, the bow is an old Elm longbow which I re worked as a Saxon short bow @ 30# for re-enactment use.
Del