Might be something to do with using degrees Kelvin rather than degrees C, then converting back to degrees C afterwards.
I'm not familiar with your equation (not being a pilot) but after pressing some buttons on my calculator using Kelvin, then converting back to deg C, I got -65.3.
This might be total rubbish!!!!
I might have converted back to deg C incorrectly, thinking about it....
No wait a minute, I did convert correctly...I think....give it a try.