RobboJock - quite right, your disc attitude will change whilst your fuselage attitude remains broadly the same thanks to the horizontal stab. I think it is more complex in auto because of the upwash caused by the RoD but I was just trying to highlight that basing your auto speed on disc attitude wasn't as good as using the ASI.
Imabell - yes I see where you are coming from with that.
Perfrej - I don't know that all aircraft will behave the same in auto at reduced Nr - it maybe that the aerofoil section (if symmetrical) will keep efficiency (the Gazelle used to be very good at it and had a specified Nr - 330 - to reduce to for max range) but more modern cambered sections might get less efficient with reduced Nr. Don't forget that as you reduce Nr, you increase the coning angle - effectively reducing the area of the disc. If your flight manual specifies a Nr setting for max range then I would use it but if it doesn't, there may be a good reason.