Hi tornadoflyer,
Yes, you've understood the interrelation correctly, although M0.87 is a bit on the high side for the 'Grand old Lady', around about 0.85 to 0.86 would be approximating LRC (flying B777s now, can't recall B747 numbers all that accurately).
The ascending order of useable Mach Numbers is - Mcrit, Max Range, Econ (these days), Long Range, and Mmo. Then of course there's Mmd, the Mach number for minimum drag, but this only comes into play at much higher altitudes than most airliners operate, where minimum drag speed is a function of Mach number, perhaps a tornadoflyer would be familiar with these sorts of altitudes
Regards,
Old Smokey