secondly because an accuracy of 0.2% so close to Mach 1 is unlikely
Distinguishing between 0,998 and 1,000 ought to be easier than distinguiching between 1,010 and 1,012 - and the latter is the known speed of DC-8. After all, it is precisely at Mach 1,000 that the shocks propagate indefinitely into undisturbed airflow? Finding how far from the plane the shocks terminate would indicate what is Mach 0,998, Mach 0,9990 or Mach 0,9950...