Best holding is to do with min fuel flow. This is normally achieved at V MIn Drag or best L/D ratio.
So the best holding speed in the 777 is normally just above the amber line, which is independent of alt. (IAS/CAS). (Same principle applies to drift down speed)
However, Drag is relative to TAS so the higher you go, the higher the TAS for the same IAS/CAS speed, and so the higher the drag. Obviously, at lower alts, the F/F is higher so there is a cross over and that is best hold.