I sometimes weigh the rats, but 90% of the time i just pick a rat whose belly is the same girth as my ball pythons belly. It is a very easy rule to follow and will always work to be a properly sizes meal.
Ball python’s belly as thick as a golf ball? Then feed it rat with a belly as thick as a golf ball.
there really is no need to weight the rat, as long as its in the right size range. Pythons eat all different size prey, and using the “thickness” rule of thumb is plenty accurate ebough as a gauge.