All of you thanks, might be useful if I explain a bit more of what I'm doing.
I'm revisiting the good old Turnback, not as a certification point, but a possible research project. My question to myself is how it might be possible to optimise flying the maneouvre, My proposed approach is to create a performance model, look to what additional aeroplane performance data may be needed to make that work and probably obtain that through flight test (for example height lost during a max aileron zero-thrust rolling reversal is not readily available data!), use that model to predict the best way (bank angles, heading changes, speeds) to fly the turnback - then validate that in flight test. Early days at the moment, I'm on lockdown so playing with research project ideas.
However, it is clear that the interval between engine failure (let's assume catastrophic and instantaneous, as that's worst case) and pilot response won't be zero, and will be significant in the turnback performance as the aeroplane is bleeding energy for as long as it's left in the climb attitude without thrust. For the time being, my model will I think leave this as a variable so that I can test its important (not actually got to coding it yet), but it would be good to have an idea from some research or research-supported-regulation (ha!) I can use as a starting point.
G