No offense bro, but please stop the recommend 10-15% of the weight thing. People shouldn't be actively saying this as it's far from the best thing to go by. If a snake is long and underfed, that 10-15% rule could be bigger than you'd want to feed and then the person could end up with issues. Best rule of thumb to recommend people is the biggest part of the snake for the same size mouse/rat.
And also... who can weight mice/rats in a store if you have to feed live.. just my thought