Gums,
Somewhere in the cobwebs of this old brain it seems that temperature is a big player to get true mach, especially up high. An SR-71 buddy of mine reminded me of this awhile back, but what do WE know?
I suggest you have a look at this interesting and well documented internet site you kindly gave us in one of your previous posts: "The Blackbird Archive".
SR-71 Online - SR-71 Flight Manual: Section 1, Page 1-135