Difference between pressure alt and true alt = 4ft per 1,000ft per ºC dev

SO 

4 x 13 x ºC dev = 1,000ft      ºC dev = 1,000/(4 x 13) = 19.23º colder than ISA because true altitude is lower than pressure altitude

OR

On CRP5 put true altitude (12,000ft) on outer main scale next to pressure altitude (13,000ft) on inner main scale.   Go to the ALTITUDE window and read air temperature next to pressure altitude (FL130) = -32ºC

FL130 ISA =  +15º - (2 x 13)    = -11ºC

Outside Air Temperature         = -32ºC

Deviation from ISA                   = -20ºC