What ISA temperature to use in the density altitude calculation?
Calculating Density Altitude

Density altitude in feet = pressure altitude in feet + (120 x (OAT - ISA temperature))

Altimeter Pressure altitude is determined by setting the altimeter to 29.92 and reading the altitude indicated on the altimeter.
OAT stands for outside air temperature (in degrees Celsius).

ISA stands for standard temperature (in degrees Celsius).

Keep in mind the standard temperature is 15 degrees C but only at sea level. It decreases about 2 degrees C (or 3.5 degrees F) per 1,000 feet of altitude above sea level. The standard temperature at 7,000 feet msl, therefore, is only 1 degree C (or 34 degrees F). 
For example, the density altitude at an airport 7000 feet above sea level, with a temperature of 18 degrees Celsius and a pressure altitude of 7000 (assuming standard pressure) would be calculated as follows.

18 – 1 = 17
17 x 120 = 2040
2040 + 7000 = 9040 feet Density Altitude

This means the aircraft will perform as if it were at 9,040 feet