You can feed by weight.
weigh the snake (grams).
This seems to be the rule of thumb...If your feeding every 5 days feed a prey item of 10% the body weight of the snake. If feeding once a week feed a prey item 15% the body weight of the snake.
EDIT
Normally bp owners dont like feeding mice because once that snake gets 1000g+ feeding several mice to a snake that size is annoying when a single rat could have fed it.
reselling a bp thats a mouse eater can reduce your market a little.