If you are making continuous loop strings it is simple to get them perfect length.
Make an extra long "trial" string, adjust it's length by threading it round and round through a loop of metal (an old bearing will do) this will give coarse adjustment, use twist to give fine adjustment to get the correct brace height. Use this trial string to set your jig for the correct length.
Use a good quality low stretch material (I use Astro flite, base on cost/performance)
The one in the pic is a bit of Ali' that I turned on the lathe, but any metal ring will do.
This also makes a very handy adjustable "long string" for tillering.
Del
PS. Don't shoot using that string, the metal ring will dent the belly of the bow (so a friend tells me
)