As Alatriste says, you are basically finding the highest allowable OAT at which you could take off given your actual take off weight. This is the assumed temperature.
This temp, when input to the FMC, will reduce your take-off thrust, (ie, the N1 value for take-off will be lower), preserving the engines. The higher the assumed temp, the greater the thrust reduction.
Obviously full thrust is still available if needed. Hope that helps.