Bill,
I think you and fowlarcher are more or less saying the same thing, and you're both correct.
When I make an endless string, I actually reverse twist it prior to serving.
For example: If 0 = no twist, and +12 = the final twist in the finished string, I serve with the string at -12.
In that case, you want to serve agaist the twist (-12) so that the serving will tighten when you twist it back to straight (0), and then continue twisting to shorten the length (+12).
With a flemish string, you can't reverse twist is past zero or it'll come apart since the twists actually hold the string together. In that case, you serve with the twists so that the serving tightens as you further twist the string to get the correct length.
For example: 0 = no twist, and +24 = the final twist in the string, I serve with the string at +12.
I hope that made sense. It's one of those things that's easy to show, but not so easy to explain in writing.