Righto, back to it.
Ok So, which speed would give you the max range then? (If you say long range cruise, explain to me why we always fly slower to save fuel).
Cost index zero. It will always have you arrive overhead with the most gas in the tanks.It will give you a speed faster than the minimum thrust required speed. In addition, as I'm sure you know, it will give you a faster speed again if you are into a headwind. LRC is pretty much irrelevant in today's CI environment, it doesn't take the wind into account and simply gives you an extra 5% speed for an extra 1% burn. Handy in the days when you didn't have Cost Index to do the maths for you.
I think I have identified where you and I are at crossed purposes Cosmo. You stated
Hence your total margin would be from top of flaps up maneuvering band, practically to MMO. This means that at max FMC predicted altitude, you have maybe 8-9 knots on each side of your bug speed (when flying just below the upper amber band) of to absorb wind variations
You have identified a range of airspeed where you feel safe operating, but the lower limit of this range is incorrect in my mind. The lower limit you suggest is the top of the lower amber band, whereas the lower limit should be min drag speed. The top of the lower amber band is some way from min drag speed. I will give you some cut and paste examples of why I think this;
The Royal Aeronautical Society has this to say.
The lowest point on the total drag curve is known as L/D max (or Vmd-minimum drag speed). The speed range slower than L/D max is known as slow flight, or the “back side of the power-drag curve” or the “region of reverse command.” Speed faster than L/D max is considered normal flight, or the “front side of the power-drag curve”.
Normal flight (faster than L/D max) is inherently stable with respect to speed. When operating in level flight at a constant airspeed with constant thrust, speed-stability ensures that any airspeed disturbance (such as turbulence) is of short term duration and airspeed will eventually return to the original airspeed if the total thrust and attitude have not changed.
So it is obvious from this that we want to stay in the Normal flight regime or we will end up in the slow flight regime which is described here;
Slow flight (slower than L/D max) is inherently unstable with respect to speed and thrust settings. When operating at a constant airspeed, with constant thrust setting, any disturbance causing a decrease in airspeed will result in a further decrease in airspeed unless thrust is increased. The lower speed subjects the aeroplane to increased drag. This increase in drag causes a further decrease in airspeed, which may ultimately result in a stalled flight condition.
The paper then goes on to say this;
Flight slower than L/D max at high altitudes must be avoided, due to the inefficiency and inherent instability of the slow flight speed range.
And
Flight slower than L/D max must be avoided in the high altitude environment. Proper flight planning and adherence to published climb profiles and cruise speeds ensures that speeds slower than L/D max are avoided.
So from that, if we are to believe the Royal Aeronautical Society ( and it's American counterparts) we should be cruising above min drag speed. That isn't really up for debate unless you consider that you know better than the folk who collaboratively produced the guidance document. That is why I asked you what min drag was for you example flight. You took a guess at about Mach 0.76. With respect, I say that taking a guess after the flight is complete is not really good enough. The Captain needs to know where the lower limit of safe flight is while he or she is conducting the flight.
Min Drag speed is usually ten knots or more ( sometimes as much as 18kts in the NG) faster than the top of the lower amber band when at high altitude. My experience is that there is usually only a couple of knots lee-way when operating at Max, not the 8-9kts that you described.
Is there a chance that the reason you feel comfortable is that you have erroneously identified a larger safe operating band than many other pilots?
Most pilots that I fly with can't identify the min drag speed if asked, yet it is only two button pushes away. The fact that it is not easily displayed to the pilots is a failing on the manufacturers part in my mind because both the Royal Aeronautical Society and the FAA produced 'jet upset training aid ' warn of not going below it.
I hope that this doesn't come across as an attack on your professionalism, rather a robust discussion and a chance for both of us to learn something. I have learnt already so thanks for that.