Good luck getting any snake to cooperate with a length measurement. An easier way is to take a piece of string and carefully lead it, length by length, down your snake until the string is as long as your snake is. I start at the head and let the string follow the curve of the spine, then once I'm satisfied a length is accurate, I hold it firmly against the body of the snake and use my other hand to guide the string along the spine. Repeat until the snake is completely measured, then measure the length of the string.
It's much easier. ;-) It's what we do in the field to measure wild snakes. If you don't think your captive ball python doesn't sit still, oooh man the wild snakes are a handful!
Go to the Hallmark store and buy a tube of wrapping paper...nowadays, they come in clear apparently....most of the time you can use this to string them through it long enough to get a quick measurement