I'd do a couple across the back, like 2 or 3 (whatever you can manage to do, since they are so small). But I don't think it really matters
you are just slitting the back so digestive juices can work their magic easier and quicker, but getting into the mouse pinkie.
If you'd like to weigh your snake, first put her/him in a plastic container with a lid, weigh it, then weigh the container and lid again without the snake in it. That way, you just subtract the weight of the container and lid from the previous weight with the snake to determine the snake on its own.