Peter wrote:
Do they add some dead weight and start the test when the aircraft is at MTOW?
Maybe they can start at MTOW*1.2, they are test pilots after all 
It sounds like you fill the tanks up, maybe add ballast, and then adjust the performance numbers by the calculated fuel burn.
Reportedly, Socata tested the TB20 at MTOW plus 30% or something like that, and (obviously) it flew. So maybe that is the other way.
@Pilot_DAR hopefully will clarify. Perhaps they build a curve using different all up mass data points and then create estimates at different density altitudes? The testing is presumably quite extensive to create reliable data.
The work equation RoC = (Excess thrust HP x 33,000)/ Mass using feet/lbs is probably a crude first approximation.
