Get a fecal done just to be on the safe side. I have a 400 gram un-sexed ball that takes a small rat every 7 days so I think you can move up
I believe if you want to go by weight for the food item take the snakes weight and multiply it by .15 and that should be the weight of the prey item (roughly)
So your 311 gram snake should eat a prey item that is about 47 grams.