I think the formula that you are talking about is to find the deviation from ISA at altitude.
EG. FL 310 + 15 / 2 = 23 (use just the first two figures of the flight level and assume that 23 is a minus figure).
This means that in ISA the temperature at FL 310 should be -23. If the ambient temperature is -20 then what you have is an ambient temperature of ISA -3 (three degrees colder than ISA).
The ISA + or - so many degrees scenario is used by the CAA in many of their exam questions.