How do you correctly pair a particular size flail mower for a particular type of cutting conditions to a particular tractor's PTO power output?
You have specified three variables, but only two of them can actually be measured:
1- Size of mower. The width is easy to specify.
2- PTO horsepower. That's easy to find most tractors.
3- Cutting conditions. How would you propose anyone specify that? Consider:
-- Wet or dry? - no such thing, it can be anywhere in between. What number would you use to specify that?
-- Type of material being cut? - all sorts of material can be cut, grass, weeds of many types, brush. How would you specify how difficult your material is?
-- Density of material being cut? - it make a lot of difference how thick the material is. Tall or short? Lush or sparse? How would you specify that?
-- Acceptable cutting speed? - as has been said by others, the same HP can cut different material at different ground speeds. What speed to you think is minimum?
-- Steepness of terrain? - steep terrain also eats HP. What grade do you expect to cut on?
Until you specify all these variables you haven't specified your cutting conditions. How would you have anyone cover these, the most important of which have no objective way to measure, in a piece of literature?
You have the answer - it's a wide range. You just don't want to accept that.