I'm still new at this, but I am useless at guessing the girth of a snake. I do weigh them, so I know what their weights are.
I once thought two side-by-side frozen rat pups were about the same size. Decided to see which was heavier and was surprised that one weighed in at 30g, while the other was 22g. Since then, I decided to throw everything on the scale, since I am apparently also useless at guessing the size of a frozen feeder. This 10-15% guideline has worked out well for me.