Attempting my first rat feeding tonight

So I went to petsmart to pick up a few rats tonight. I didn't want to commit to an online order while still trying to switch over.

Using the 10-15% rule, at 55g for a roughly 550g snake, these rats should be perfectly sized right? They seem big, but it could just be that she's really out grown the mice.

How long does the 10-15% rule hold true for?

I believe it holds true all throughout the BPs life. I've never weighed the rats, so I can't be sure, I just go by look. If you put it in and think "there is absolutely no way" then take it out and get smaller. If it looks pretty close to the same thickness as the thickest part of your snake it's fine. It's also better for it to be slightly thicker than the BP than a lot thinner. Just be careful about not going too much thicker, only slightly if it's between that and a much smaller prey!