Driveshaft length tolerances +/-?

Well, I haven't made it to a shop just yet to fab a DS for me. I was just thinking of shopping around for the correct length to avoid having anything cut down. I know the length that is specified for my build, I'm just wondering how close I need to be to that number for it to be acceptable. Thousandths of an inch? 32nds of an inch? 1/4 inch?
check. call a couple and ask. off hand i'd go with install yoke all the way, pull back 1/2" and measure