One reason for such a wide range of hp recommendation is the dumb user. Not calling you dumb. Have you seen the post on the operator who got killed by a JD rollover due to no rear
ballast and sued JD and won? The owners manuals are very clear on safe loader use. But dumb or in to big a hurry to read the manual or the lack of common sense happens so the companies have to try to protect themselves from those people. In all honestly we all can be one of those if not careful, me myself.
For starters it is a pto powered implement so you are working with the PTO hp and not the gross engine or net engine, only the pto hp. Now the range is due to the wide range of what can do with that mower. A flail mower can be operated in a wide range of circumstances. You can travel very slow or say 8 miles per hour. Big difference in the hp needed with the speed difference so if you will be mowing fast high hp needs. The density of what you are mowing,thin less hp of course thick more hp. Same with the height of what you are mowing, short needs less hp than does tall as flail mower does mulch what you are mowing more so than a rotary mower will.
I am in process of matching a new flail mower to a new to me tractor and with my experience am going with about of 75% of the max hp recommended. Why, for one thing I want that wide of mower to reach my ditch banks will be mowing and the other is the small amount of time I will be in heavy cutting don't mind traveling slow. Matter of fact I suggest very much from safety with any type of mower to cut slower in heavier crops.
Most of my flail use has been on boom mowers and very little with a flail mower on grass. I am very use to running in the lowest gear I have and still feel like I am flying due to all the operating to miss the tree that is huge to the changing terrain tilting mower up and down.
You will find the same process for any implement that tills also. Take a simple disk, the trash being cut, the type of dirt, the moisture level, the speed you want to run and the type of tires and ballast it has. Way over kill with hp is a waste but way low in hp is work not done and great aggravation.
Oh just thought of one big thing to consider, the result you want. Not with all implements for certain but with mowers or tillage their usually a speed at which if you exceed no machine with no amount of hp can produce the results you want. Any mower has a max speed for good results and tip speed comes into play there. You get that from the mower company. In all this you be sure you then run the tractor to get the recommend pto speed for the mower. The pto must be at the rated speed and you use the gears to get the right ground speed, not the throttle.