I  like to   find  the  highest gear  the  tractor  can pull it  wot,  without  excess   exhaust temp...   which may be  1-3 mph  faster than  need be.   then  pull the  throttle back to  meet  the  needs  of the  tool.   If  it can handle  a full  round of  Wot, which  should include hills,  tough spots.. throttling back is  a snap.     It all depends  on  your  transmission,  and if there is a  gear that 'fits'!!    Seems like  just about any tractor  needs more  'gears'!

Since   torque is  the thing that moves  the  tool  through the  soil,  and  HP is just  a calculated number..(torque @ constant crank rpm)...      I have found   that   those nifty  torque charts  are pretty dang  accurate on finding the  'sweet spot'.. 

Inversely,   if  your tractor can't pull it  with the  throttle set at  the top of the torque curve, *typically  200-300 below  high idle*    it sure can't pull it  WOT.  (same gear,  same work load)