The essential method is to use the airport analysis (r/w, obstacles, pressure altitude etc.) and the TOGW, and determine the maximum OAT for a legal TO in this condition.
This is the max FLEX temp.
You then set thrust equal to the rated engine thrust for that FLEX temp. (For V2500, set EPR for that FLEX OAT; for CFM56 use the sliding-scale N1 chart entered for both actual and FLEX temp)