Re: How to determine sling length ?
Oh, they're rock solid once you get used to them, and in conjunction with a front bag, you'll do better than you likely would off a bench. Still, they do take some work. Remaining in position for the time you likely would in a hunting situation would be a bit of a problem, at least if you're running it like we do in competition. That's why I mentioned the glove. The sling will cut the circulation off to your non-firing hand if you don't have one, and you probably won't want to stay there for more than 25-30 minutes even if you do. I routinely come home black and blue in my shoulder from smallbore matches, and with very little feeling in my left hand. The brusing isn't from recoil, of course, but from how tightly the rifle sits in the pocket of my shoulder. And that's with a heavy shooting jacket. Yeah, they're tight.
Glen Zediker did a couple little (really, very small!) books about this. One is called Slings & Things, and the other is called Service Rifle Slings. I think they're both available from Midway, but I'm just about sure you can find them at Sinclairs. They deal with slings and their usage, talk about all the different materials, how to set them up (there's several different ways; all work, some better than others). Considering the price of a good M1907 sling like Tam's or Turner's, might be worth your while to take a look at these. At the very least, it'll give you a better idea as to what you're after here. Check 'em out, and don't be afraid to ask questions!