I just actually found an awesome way to do it, I measured all my snakes this way and got accurate measurements.
Serpwidgets - Snake Measurer
I took a picture like this
Basically you just take a chunk of the measuring tape, say 8-16 and list it as 8 inches and then put points along your snake and it tells you how long it is. All my snakes were measured correctly and it was really simple to use. String is extremely annoying to use and if you have snakes like mine just does not work.