The trouble is there are heavy offset disks that take 350 hp tractors to pull less than 20 feet, and there are little finishing disks with the blades worn down to 14-15 inches on easy ground that don't need 40 hp for a 12 foot disk......
In general I'd put a 10 foot average finishing dik behind a 50 hp tractor. Would be a heavy pull in spring in plowed soft ground, and wouldn't hardly know it's there in hard fall bean stubble.
If you have new, bigger blades, or you are weighting it down and trying to work hard with it, a few more hp don't hurt.
Then again, I recall pulling the 12 foot trailing disk with the 36hp Farmall as a kid in just the right conditions and getting along with it.
Simple and easty answer, eh? :)