May I submit that for the DC9-30 to DC9-50 series of aircraft, Mcrit is M0.73, as indicated in the Airplane Performance Manual.
Drag rise between M0.73 and M0.76 is considered acceptable for normal MRC / LRC / Econ operations, whilst aerodynamic characteristics between M0.73 and M0.80 are considered acceptable without compensation. Between M0.80 and Mmo, Mach Trim Compensation is required, and provided.
The DC9 / MD80 / MD90 / B717 family is very large, and if I have commented upon a derivative outside the scope of the discussion here, I withdraw and take a back seat.