A couple of considerations that I have found in the last few weeks include:
The diameter of fast flight strings are much smaller than dacron. Prior to switching, my normal strand count for my set up required 15 strands of dacron, which fit the Bohning Classic Nocks on my wood arrows very well.
From switching from Dacron to Fast Flight Plus, I had to use 18 strands to get my nocks to fit the serving.
Prior to using 18 strands, I made a three bundle 15 strand string which did not fit. I then tried sixteen, which also did not fit. The 18 strand fit good.
My point is, don't get discouraged. It may take some tinkering before you get it right. You might want to tell the guy who makes your strings what types of arrow/nock set up you have. He may know what serving/string combination will fit your set up.
good luck