I've always followed the guideline of 10-15% of the snake's body weight every 5-7 days, until the snake is about 500 grams. 10-15% of 282 grams is 28.2-42.3 grams, and it looks like you're feeding right in that range. So what you're feeding sounds good to me.

A large weanling rat does sound a bit small to me for a 900 gram snake (I have a 500-ish gram male on small rats), but if that's what he'll eat, then I guess that's what I would feed him.