Soopahmahn
Tu ne cede malis
you need to measure up to the tuner, i suppose, and account for at least a turn or two to get the string wound up. so that's 30" + length to tuner at end of headstock+ a few inches more to wind it around
there was a thread on this a few months ago, you could try to find it
I find that 1" is enough to get two solid winds out of all but the heaviest of strings, maybe leave 1.5" for your seventh and eighth strings.