It is not difficult to calculate the N1 setting, if you already know the limit N1 and the flat-rating temperature.
For example, if a certain high-bypass fan-jet is flat-rated to 32°C at 98.0% N1, then you could determine the limit N1 at any LOWER temperature. The conversion factor is : square root of the ratio of OAT and flat-rating temperature (all temperatures must be in Kelvin).
eg. at 10°C, 98.0% x sqrt((273+10)/(273+32))
= 98.0 x 0.96326
= 94.4% is the limiting N1 (max T/O power)
For deration, example OAT is 15°C and flex temperature is 45°C: if the limit N1 at 45°C is 93.3% N1, then at 15° the N1 should be 93.3% x sqrt((273+15)/(273+45)) = 93.3% x 0.95166 = 88.8% N1
The flex N1 is 88.8% at 15°C (flex temp is 45°C)
You could make a spreadsheet showing the derated N1 against OAT, and take it with you for flight. Use it to cross check the N1 you get on the engine instruments (if your engine is EPR rated). But remember to make separate charts for different pressure altitudes (sea-level, 1000', 2000', etc)
[This message has been edited by Old Dog (edited 20 November 2000).]