You are on the right track, and it's good to question her size. I'd say she is on the lighter side of what could be considered a healthy weight. That being said, BP's, like any other living animal, grow differently. You could have a 500g snake and a 1300g snake, both born at the same time, and both completely healthy.

If I were you, I would invest in a kitchen scale that measures in grams, if you haven't already. You want your prey to be 10-15% of the snakes weight (Like you already know). I would feed every 5 or 6 days. She could probably start taking small weaned rats at this current size. I bought a bag of frozen weaned rats and they varied from 33g-48g. If you only have or can only get mice, feed her two or three (or however many equal up in weight to the 10%-15% mark). If feeding multiple prey items, wait until the first is swallowed into her belly. That's when she'll be looking for more. People will suggest rats over mice because it's easier to just feed one prey item for the most part. It's up to you what you want to feed, as long as the snake is getting enough.

Once you start giving her that 10-15% feeding weight, she'll start putting on weight. It's important not to overfeed her at this age/stage. Yes, she's a little underweight, but the best thing for getting her back up to a healthy weight is a regular, consistent, healthy feeding regiment.