To many variables must be the same, or very close together for a rule of thumb you list to work or apply
Example is the HP required to overcome all of the losses...then what HP is required to do the job of moving and keeping at speed for your setup
That HP min should be at the min RPM found on the torque/HP curve graph...therefore throttle setting. Note all of these graphs is at WOT
Losses...pumping, frictional, drag, sail area, inertia of mass, rolling, etc, etc and note that they all increase as the speed increases
-Ben
Picture of my rig1996 GMC SLT Suburban 3/4 ton K3500/7.4L/4:1/+150Kmiles orig owner...
1980 Chevy Silverado C10/long bed/"BUILT" 5.7L/3:73/1 ton helper springs/+329Kmiles, bought it from dad...
1998 Mazda B2500 (1/2 ton) pickup, 2nd owner...
Praise Dyno Brake equiped and all have "nose bleed" braking!
Previous trucks/offroaders: 40's Jeep restored in mid 60's / 69 DuneBuggy (approx +1K lb: VW pan/200hpCorvair: eng, cam, dual carb'w velocity stacks'n 18" runners, 4spd transaxle) made myself from ground up / 1970 Toyota FJ40 / 1973 K5 Blazer (2dr Tahoe, 1 ton axles front/rear, +255K miles when sold it)...
Sold the boat (looking for another): Trophy with twin 150's...
51 cylinders in household, what's yours?...