I have to say that this all seems terribly complicated.
If I have to play this game, I can do it as well as the next man and produce complex formulae and solve them with a calculator.
I can also use a circular slide rule nav computer if I need to.
But, once above 36,090 pressure altitude (which we are) then ISA is -56.5ºC - at least, until we get to about 65000 feet.
So Mach One = 573 knots TAS (or, more practically, a bit less than 600).
So your Mach meter becomes a 'mile-a-minute meter' - ie, Mach 0.70 is 420 knots (7 miles a minute).
In the air, nobody's going to argue about 7 knots (the completely accurate answer).