Yes, once again a thread has completely gotten off track into the world of something that was not asked.
Let's say it this way. Company A has a general, albeit unwritten policy, of happily recording DDR data if it's a pass but discounting DDR data if it's a fail in favor of hand plotting long hand data on the chart from the RFM.
This goes on for a long time until the day something bad happens. Aircraft departs, one engine kacks just after CDP and the crew tries in vain to fly it away on the remaining engine. However, this engine has not been doing too well on the old power assurance checks and routinely failing according to DDR data. But, as chart data has been used to calculate results, a pass has constantly been achieved, thereby allowing that engine to remain in service.
Now here's where it gets sticky. Murphy being what he is, the crew tries in vain to fly away but find they just cannot get the damn thing to climb (remaining engine is just not up to the task).
After the aircraft is recovered and the inquiry begins, some clever aviation litigator keys on the unwritten policy of Company A and asks the million dollar question, "Uh sir, can you please show me where it states the procedure you have been using is legal?"
We can talk about the inner workings of the system and why they do what they do until we're blue in the face, but truth is it won't matter a flick after you've bend the bird.
Is the PIC going to cop it yet again, or is blame going to be laid at a higher level within Company A for having allowed what is essentially a non-documented procedure for recording and applying power assurance data?
At some point, there has to be one and only one way of recording and applying this type of data. If not, why even bother with power assurance in the first place.