@ Selfin,
That formula was for mental calculations for rather slower airplanes (DC-4 in particular when the ASI was in mph and at 10,000 ft, 200 mph indicated were worth 200 kt true ( you see what I mean )
For faster speeds, when one enters the Mach territory ( First plane for me was the Convair 580 /540 whoch cruised at ;55 or ;60 M )
In this case, the formula became
TAS = M x 600 . Exact at OAT = -35°C. just add - if the temp is higher - or substract - if lower - 1 kt per degree different from -35.
Ex :
M = .75
OAT = -42°c --> (-35 - 7 )
TAS = (.75 x 600 ) -7 = 450 -7 = 443 kt
The error will not exceed 1 kt !