The different species in the Morelia family don't actually all share the same girth to length ratio. It varies depending on each specific species.
Deciding the size should be done depending on the girth of the snake. If the snake is over or under weight, like you said, it wouldn't be a good reference but you would adjust it accordingly to what that specific snake needs. Using a general feeding rule in that situation could be harmful to the snake. For example if say a yearling should be taking a small rat for it's food but it is really underweight, you wouldn't just start feeding it a small rat to bring it up to weight. That could be harmful to the animal. Feeding it a smaller prey more often would be the safer way to bring it up to speed.

The reason there is no definite recommendation as to what size prey to feed a snake is because there are other factors to consider then just the size of the snake. Is it a breeder? Are you just maintaining weight? Age of the snake. How often you feed. General rule is every 7 days but you can safely feed less often depending on the age of the animal and size of the prey.

There are guidelines people can follow but there will probably never be a solid specific guide to follow.