Henry
Henra, that number, in the 60% range was a calculated value. To my personal knowledge, transient power off flight test experience goes down to around 80%. Talking about the in flight data. Power off Nr may well go a bit lower during the landing phase ( although, one better have the machine close to the ground and the rate of descent within limits by the time Nr has drooped to the low 90% range because, well as you all who do these landings know, your ability to produce more than one G, and thus decelerate vertically, is about gone by then).
So, I completely agree with your, may I say, " experienced reluctance " to use that range as usable. I did not mean to covey that idea.