For bow strings, we use a security of 4 (3 if we desire thin string), but I don't know if that's the same for crossbows.
So for bows : you take a linen or hemp string, cut a 2 meters long part and you test the resistance of this string (with weights, water bottle or portable scale). you have to know which weight the string break.
the longer part that resist you put a weight once more to test the resistance... make it 5-6 times to know if the resistance change a lot from the first break and the last one.
For endless loop string, that's better to make the average from the 2 first breaks, for flemish string, you can make the average of all test result.
When you know the average resistance of your string, you know the draw weight of the bow that you multiply with you security (4 for bows, 3 if you change string often) and so you can know how many linen strings you need to make you final bow string.
B-50 resit to 50lbs normally (which is never true, more about 40)... for linen, that's generally between 5 to 15 lbs. hemp strings are generally bigger.
If you have a 10lbs linen string and want to make for a 50lbs bow, securityx4, you need 20 little strings for your bowstring !
That's a bit more complicated if you make a flemish bow string because we try not to make more than 7 little strings for 1 strand and it's hard to make bowstring with more than 4 strands.
Not sure it helps you...