With the new, low stretch strings, I think the amount of tension is not as critical as it is with a bulky B-50 string.
What's worked well for me, and I've ordered strings from several of the sponsors on here, is measuring off the bow.
I have two wooden blocks with 3/16" brass pegs in them (OK, not 1/4"). I clamp one to my workbench, put the string over the peg and slip the other loop over the other peg, pushing the string tight and then clamp it to the bench. Then I tap the one of the blocks with my leather mallet until I get a "fair amount" of tension on the string until it sounds about like it would if plucked gently on the bow. I've ordered the string to that length and told the string maker how I measured it and have yet to be disappointed.
Probably not the way the string maker would prefer to get measurements, but as long as he knows how I measured it, the string has always come the way I want it.