ChristiaanJ - Upon reflection, I'm not sure that it was the Relight Envelope, but the shape is similar to an engine performance relationship where the 'horse's neck' was referred to.
But the Relight Envelope was established by demonstrating relights during flight testing, on the basis of a theoretical envelope.
There would have been much discussion on the build standard and state of deterioration of engines involved and so on.
Sorry I haven't a copy of the Envelope, but I'm sure it will be in the Flight Manual.