I do not understand this part:
There is no external 1.1 V reference connected to AREF. Internal reference would be 2.5 V or Vcc (5 V).
Ho do you arrive at this factor 0.442?
By the way, doing float arithmetic on a microcontroller without floating-point unit is very slow.
Try using (long) integer arithmetic like
Division still takes a lot of time. By modifying the equation like this: 442 / 1000 = 453 / 1024 (approx.), and noting that 1024 = 210
and also noting that a division by 210
is equivalent to shifting the divisor 10 places to the right you get this code:
temperature = (res*453)>>10
Note: these are mere suggestions to speed up the code which is not really necessary here but may come in handy once your code grows larger and slower.