It depends on your definition of small rat. I wouldn't feed something around 200 or so grams a "small rat"- I would feed them a fuzzy rat with eyes open or an adult mouse. Don't worry so much about the percent rule, just stick to what will fit your snake's individual girth. And if it seems a bit big, remember snakes have been taking down things a little bigger than they should have since the beginning. Slightly bigger is okay as long as temperatures are correct in the enclosure. Here is a picture of a fuzzy rat I would suggest:
EDIT: I didn't mention slightly bigger is okay for the occasional big fuzzy, but try to stick around the girth limit.