I had an old Honda 175 rated at 20HP. I'm always reminded of that 1968 CL175 at 20HP that would do 75mph easily on a flat road, and over 80mph with some tail wind or any down hill. Most motorcycles are probably using the same range of 20-25 HP when travelling at 70mph. All that extra HP is just for passing, speeding, and showing off. The performance of that 1968 Honda CL175 was pretty impressive for such an old little engine. She wasn't that pretty, but I got it running well, and then sold it and it's donor CL175 carcass.That figures out to require 20hp to go 70mph and 29hp to go 80mph. And that figures out to be a 45% increase in power requirement. Your method produced an estimate of 48% increase in power to go from 70mph to 80mph. Amazingly close I think. Maybe I'll remove my windscreen and go try it - or not. No wonder the gas mileage plumits at high speeds.
I should have put PR4's on it.