A simple formula for ram rise is (TAS/89.1)^2 and this is exact, with TAS in kts.
Of course this means you need to know the TAS.
At -65C, the speed of sound is about 562 kts.
So, at M0.78, TAS is 562*0.78= 438 kts.
So (438/89.1)^2 = 24.2C.
So the TAT is -65C + 24.2C = -40.8C.
The altitude is irrelevant, only the OAT and TAS determine the ram rise.
Furthermore the TAT is the temp that is actually measured by the probe, and the OAT you see on the screen is calculated by the ADC's by the reverse of the above calculation.