The usual double draw length plus 10 per cent applies for a target crossbow but here's the thing, pulling a string back 16 inches two handed while bended over is really unpleasant, I suggest you try picking up an anvil 16 inches off the floor without straightening up. A 20 inch draw would mean a 50 inch long prod and that's pretty unwieldy. My current build has a 28 inch long prod and that's pretty wide, Early wooden crossbows had longer prods and longer draw lengths, though, anything from 80 to 120cm long but I think they only drew to 12 inches or so, like this one
For hunting you need to keep the bow drawn for a really long time, so mid and late medieval bows were massively under stressed by having huge draw weights but drawn on a short draw, so a 28 inch wide bow drawn only 9 inches. This gives a power stroke of 6 inches resulting in a crossbow being equivalent to a longbow of 1/3 its draw weight. A 120lb crossbow is barely equal to a 40lb archery bow.
Sinew changes everything, you need to go with a shorter bow to get the sinew working, it also allows you to keep the bow drawn for longer, my 28 inch long yew and sinew bow draws to 11 inches and hits really hard at just over 100lb but I have mounted it in a much shorter draw-length tiller at 8.5 inch draw length (65lb) with a 5 inch power stroke. It is really unstressed so I don't have to worry about taking my time to aim.
