For the very low temp, we use SAT in the air...
If during Climb or Cruise the SAT is below -40 Celsius, No Anti Icing required. This is because no 'wet' particals remaining in the air which can freeze to your aircraft..
In my early days on the 737-200, we didn't have that fancy FMC with the SAT.. I remember we used a rule of thumb to calculate the SAT, since we only got the TAT indicated. I'm not very sure about this rule anymore since I didn't have to use it the last years.
Must be something like TAT - (Mach * 40) = SAT
Correct me if I'm wrong, but I'm going to try this afternoon if that works..
So, TAT indicating -12, Mach is -.74
Roughly: -12 - 29 = -41 (seems correct)