Not only is a chain measuring tool a waste of money, it doesn't properly measure chain elongation. If you're interested, here's a good writeup on it: http://pardo.net/bike/pic/fail-004/000.html

I use and recommend an 18" steel machinist's rule. These are thin stainless steel with fine scribed markings which makes accurate measurement easier. The pix below show a chain exhibiting 1/16" elongation over 12" (13"-1"= 12"). I start from the 1" mark because the scribes on both sides of 1" help center it on the pin.