Hello!
I just visited
http://www.longhair.org/measuring.htm and it says:
"Begin by standing up and pulling all of your hair behind you. Let your hair fall unobstructed down your back. Stand with your side facing a full-length mirror, if possible. Stand as perfectly straight as possible and place the end of the tape measure in the middle of your forehead where your hairline starts. Let both your hair and measuring tape fall behind you along the line of where a middle part would be. The measuring tape needs to be in the middle of your back. Whether your hair is all one length or layered, measure to the end of your longest strands. It may be impossible to read your hair measurement from the mirror. So while still holding the beginning of the tape measure on your hairline, feel for the spot where your hair ends. Grab the tape measure one or two inches above that spot, using your thumb and fingers as shown below. Please make sure that your hair and tape measure are both perfectly straight when you grasp the spot. Once you have the exact spot on the tape measure grasped, you can let go of the beginning of the tape measure. Now simply pull it around for a quick reading."
The website also has pictures explaining this if you are confused as well as a lot more interesting and great hair advice and tips!
Hope I helped. ;)
Whitney