Back when we used to plow, we used to figure 16 hp per bottom.  That was on the high side, according to our neighbors, who plowed with smaller tractors, smaller bottoms, and plowed shallow.  Used to be a lot of argument about that back then, doubt if it changed much now.

We had bottom land that pulled hard, and lots of variable soils that were dark and light in the same round.  I remember shifting a lot, especially with the torque amplifer, sometimes with the gear, in different soils.

Our 1066 pulled 6-16's, 856 pulled 5-16's, and sometimes it was plenty, sometimes we went pretty slow.  Used to pull 3-16's with the 400 IH, 2-16's with the M Farmall, with the M in third gear, sometimes second.

There are so many different soils, and different conditions such as wet and dry, the rule of thumb of 16 hp per bottom didn't really mean a lot.  We sometimes plowed with chains on the land side wheel, as the wheel in the furrow had better traction. 

Also, if the plow is on land, we used to say we lost a bottom of pulling capacity, so you would have to figure more horses per bottom if it were an on-land hitch.