The only help I could offer about the Concorde is that Mach crit is not a published number in the manuals (well not that I've noticed!!). If we have to cruise subsonically we do it at M0.93 (M0.95 if the autopilot is engaged in alt. hold). The latter is interesting because approaching M1.0 and over the transonic speed ranges the elevons are kept very busy by the active fly-by-wire system due to the shockwaves "dancing" around, often assymetrically (sp?) across the ship. The autopilot in alt. hold is considered smoother and therefore the increase to 0.95. Hand flying through the accel. highlights the constant changes in pitch and roll trim for this reason until about M1.4ish (although the handling is always precise and accurate at any speed - a joy, in fact). Once above M1.7, reheats turned off, it is like a train on a track. As to mach crit by the above definitions - I would guess something above M0.9 but I don't know.....