Originally Posted by
Greedy
The problems with this are that it assumes ... accurately setting zero thrust.
If you were able to accurately set zero thrust, you'd be able to accurate also know the thrust for level flight, and simply fly at a trimmed speed.
In fact, that's the more common approach - to ASSUME that thrust is known, and use that to define drag. After all, in terms of usable performance numbers, what you want is the condition with thrust, so even if you could somehow define drag directly, you'd still need to combine that with thrust data to give some kind of useful performance data to the crew.