That would be 100% of its size. If you could 10 inches you can expect it to stretch 50% of that which is 5 inches
Duh, of course you are right, I've forgotten how to do basic math, I'll blame it on my momnesia.
Okay so ideally I should just cut it to the proper lenght (i.e, head is 20 inches, cut 20 inches) and then overlap it by half an inch? Now is that half an inch on each side for a total of one inch or a fourth inch on each side for a total of half an inch?