Use the 10-15% rule until they reach about 500 grams. From there on it's basically one small rat once every 7 days.

I agree. The prey vs snake eyeball size test usually fails me too. I thought I was doing great when I started out until I bought a gram scale and realized I was feeding one of my little guys 25-30% of his weight. Surprise, surprise he soon went off food, and sent me into a new challenge of wondering what was wrong. Err on the smaller side, and they will feed consistently, and ultimately grow better, and in a healthy manner.