or .86 mach at 42,000.
Actually grounded no! The whole point of this discussion is that if the tracking capability relates to a groundspeed rather than an altitude rate (i.e 330 ft/sec) then your statement above won't be true. Even if the the a/c were to be within 2500ft of the ground at 42,000ft its speed would exceed the tracking capability and therefore be NCD because 330ft/sec equates to only about 195kts.
Please RTFQ.