John_T,
Of course, but this was after his time, not only did we discover this wrinkle with the SP, but even the usual Boeing performance guidelines for assumed temperature de-rate calculations could, at best, be described as "ambiguous". At least Wal's graphical presentation of performance made it possible for a pilot to see the problem, the tabulated data preferred by many operators would have made it quite hard for a pilot (even with an engineering background) to see the problem.
It took me a while to work out the error,(resulting in excessive de-rate) but finally, when the penny dropped, it earned me a couple of free beers from the then incumbents of the performance department.
I took a very personal interest, seeing as I was flying B767-238 regularly out of the critical runways, at the time.
Tootle pip!!
PS: Wal's "Performance of the TurboJet Aircraft" is still one of the best primers on SFAR422B/FAR 25 performance, after all these years, a pity somebody has not updated it for recent amendments to FAR 25, and re-issued it.