This thread seems to have opened up a "can of worms"!
Downloads by the tails significantly decrease the ease of an aircraft's progress though the air. The value of the download varies considerably to preserve natural stability over the speed (IAS and Mach No) cleared envelope and as cg position changes.There are a few other minor variables.
Do I detect that predicted tail downloads are sometimes being used as a variable in performance expectations by the addition of the download to the aircraft mass to produce an apparentl weight?
It's a while since I have been active as a TP but in my day when I did a measured take off and other performance tests I always assumed that the aircraft designer had taken into account the tail down loads when calculating the aircraft's weight being supported by the wings and any body lift.
The onset of acceptable artificial stability and an increasing emphasis on aircraft efficiency must be impelling us to accept yet another variable in flying with minimum drag.