Alex Whittingham . . . .The normal RPM term (N) in an engineering analysis of thrust output involves sqrt(temperature). To make the numbers a little more sensible, it is usual to reduce the value to a reference sea level condition by introducing standard sea level temperature into the equation. This changes the presentation by using the temperature ratio, normally designated theta. I am not sure that mach number is relevant generally .. I suspect that the RPM limit more usually is a blade centrifugal loading/tip clearance consideration.
As to where the thrust break temperature might be for the advertised flat rating, this will be solely a matter of commercial and technological compromise both in the flexibility offered by the engine manufacturer and desired by the airframe manufacturer. There is no significance, per se, in any particular deviation temperature.
. .Keith presumably has a RR background .... ? The alternative throttle setting procedure seen often with US engines is to predetermine the relevant limits and then set the throttle (power lever or whatever term you might prefer) to achieve the relevant limiting condition.