Why does nobody bother to read what the poster is ACTUALLY asking!!!!
There is no such thing as density margin in relation to thrust.
When calculating thrust for assumed temperature take off method. The N1 is corrected for the difference between the actual and assumed temperature. This means you get exactly the same level of thrust as if it were the assumed temperature.
The density is corrected out.
The margin appears as lower ITT/TGT. This is entirely dependent on the difference between actual and assumed temperature and the flat rated temp makes no difference.