When guessing the age of a young snake- first look up the average length of hatchling for the species. Then realize it takes a long time when a small snake is eating young (small) prey to get much length- from there it's just an educated guess, lol. At least BPs are pretty big for hatchlings. I've raised many snakes that start much smaller, like corn snakes, & they have to eat pinkies for a very long time- hence, they grow very slowly until they get into larger prey. The smallest snakes I've ever raised were desert glossy snakes- about 7" hatchlings- talk about needing patience.
I figured (hoped) you were being gentle about measuring your snake, but not everyone is or realizes how easy it is to injure a small snake, so my response was intended for general information to all. Some snakes will stretch out on their own when placed at the baseboard of a wall, btw.