I agree with Hivemind. I recently made a three point chest harness back quiver for my brother and I measured from where it would attach to the ring over the shoulder he waned it slung over and to what point it would attach to the body of the quiver. I then measured around his chest to determine the other strap length, adding several inches to each side for the attachment to the ring. I made it so that the chest strap can be adjustable around the body with a buckle end. However, if you are producing these to sell at a convention or a fair I would suggest as Hivemind did to make them long and then you can trim them down later. In fact you can still sew everything together and then trim the free strap end to the proper length. However for a truly custom job, I would not attach the ring to the straps until you had taken someone's measurements and then you can hand sew it together and they can pick it up later. That's just the way that I would do it, for what it's worth.