In the wild the natural prey is only about 3-4 inches long max. They dont need much to be healthy. We try to only feed our snakes rodents in the size of 4-6 inch. This keeps them from getting to fat but they dont starve. For ones that are a bit bigger well feed 2 rodents at a time. But as for our big 4000g girls we just try to find the biggest rats we can lol.

Ive never heard of the 15% rule. Ive always went with the girth rule. As long as the rodent is as big around or smaller than the snake. Its not to big to be fed.