making my first crossbow string

My yew and sinew lath has been curing for 6 weeks, so its time to tiller it. I am hoping the final draw weight will be in the region of 150lbs.  I have made about a dozen flemish twist bowstrings but never made a crossbow string.  So, how do I measure it, and should I go with flemish twist or endless loop?  Also, I use 18/4 barbour linen thread for my bow strings and was planning on doing the same here, any thoughts?

For medieval crossbows, you've got a choice. Western and southern European crossbows usually use a continuous loop string.  Central/eastern and northern crossbows, particularly those with horn/sinew prods mostly use a knotted string that is double-thickness at the ends.  You should be able to find videos online that show the process.  I got the hint from Harmuth's "Die Armbrust' pub. 1986.  Making continuous loop strings is very straightforward.  Making the knotted ones takes some practice.  Geezer.

Geezer here with a correction: I did a quick search online and couldn't find anybody with a video showing how to make a properly knotted string.  One guy was using classic eye-splices, like those used at sea.  That might work if you do a reasonably thorough job of the splice.  Harmuth's "Die Armbrust' and one of Richter's books (I think Die Hornbogen Armbrust) have illustrations of the proceedure, by which one essentially makes an oversized continuous loop string and then loop the ends back over themselves to create slip-knots.  Then you have to serve the knots carefully to keep them from slipping any more.  I have done plenty of those strings... they take more time and sometimes you do get a bit of knot-creep, but they're strong and have the right look for medieval bows.  Geezer.

Thanks Geezer, I've spent a while researching and found the illustration you mentioned in Dei Hornbogen Armbrust, and the videos by Jos von Eichsfeld, the one question I can't find an answer to is the string length to lay out - presumably if I lay out the string at the distance of the lath from nock to nock (DNTN)then when I put in the slip-knots it should take the string to more-or-less the correct length (1.5" less than DNTN?) and I can twist in any slack that might remain??

Oooh, that's a hard one.  To some extent it varies by the thickness of the string as well as the  string length and size of the eyes.  Generally I'm making 27 inch strings for 'industry standard' aluminum or steel prods (like those made by Garvin the Slow or Darkwood Armory) For those, I make an endless loop string about 1/3 larger than a standard continuous loop string.  To some extent, you can alter the looped-string size by sliding the knots or twisting the string overall when finished.  Beware, over-twisting a string will end up breaking surface-fibers and make the whole thing fail.  So try starting with a 1/3 oversize skein of string that isn't very valuable.  Experimentation will result in something that works, but you are likely to waste a few before you have it down.  Geezer.

1/3 oversize; thanks I would have been woefully under-size, saved me again!! If if ends up slightly over, its no loss as I need a bastard string but if it ends up under then that's a different matter. As an aside, I have found that an over-twisted string unwinds at the end of the stroke, dramatically decreasing string tension and losing a noticeable amount of power.