Life on top
The margin you are talking about comes from difference between take off speeds in IAS and the TAS at which the aircraft is travelling because of higher density due to lower temperature but the example you have chosen is not correct. If you choose ass. temp. of 50 degrees for OAT of 35 degrees( which has to be higher than Tref). You will have performance margin as compared to the take off done at actual OAT of 50 degrees. Reproducing from my earlier post:
Because we are operating at an actual temperature that is lower than the assumed maximum, true airspeed likewise will be lower.
Because of this true-airspeed effect, we enjoy a great deal of cushion between what the airplane must do and what it actually is doing. We are, in reality, using less runway and achieving a higher climb gradient, or obstacle-clearance margin, than if the ambient temperature was at the maximum for that same weight. Depending on conditions, the effect can be considerable — on the order of several hundred feet in field length. The benefit increases as the difference between the actual and the assumed temperatures increases.