In my opinion, your BP looks fine as far as weight goes. BPs are supposed to be more "rounded" bodied snakes.

I've never heard of this skull size thing...feeding between 10-15% of the snake's weight or a prey item approximately as big around as the snake is, is an excellent way to judge prey size. It doesn't have to be exact, and if the prey is a little big bigger around or smaller around than the snake, no problem. They were made for this kind of thing.

I, too, max my feeders out at around 100-120 grams. Every now and then someone gets a bigger one when one comes along.