ColorMeGreen that sounds like a well reasoned rule. I know it works for me. My tiller is 3 and a half feet wide which would make the requirement around 17 and a half hp. I have around 17hp at the PTO and wouldnt want to go to a bigger tiller for breaking sod with the soil I have. Maybe if I lived in one of those places with deep loam or a sandy soil but not in this Pennsylvania rock and clay.

From what Ive heard you may want a larger figure if your in a place with many rocks like New England.