BIG BORE FOUR -
Remember we were never at MTOW for long in this machine! Initial subsonic cruise ex-LHR was 280-300. In the case of engine failure enroute, it usually ended up in the mid-30's (with its subsonic 4 engined contemporaries).
The process explained in the extract from the OM is worth thinking about...you fly Mach, but the IAS still plays a dominant role on drag as you climb. Most unlike conventional types.