I never use weight to determine prey size. That is not to say it is wrong, I just don't do it. I pretty much feed using the body width method. If what I have to feed is a little small I will double up or feed twice in a week. In my experience, for young snakes, feeding about every four days works well. For older snakes I will double up on the same feeding. These are all generalizations though. As long as the snake is keeping good body proportions and gaining weight everything is fine. If the snake starts skipping weeks and you want it to eat every week you drop down a size. They do not need as much food as we seem to think they do. We overfeed in captivity.