Use the current altimeter setting at a surface airport within 100 NM of your location and that will be accurate enough for cruise calculations.
Here is the reasoning behind this. The airplane is flying a pressure altitude of (for example) 8,500 FT. It is not going to be the true altitude above mean sea level as the pressure can change based on many factors. The airplane senses it is at a pressure altitude of 8,500 FT.
Since we are properly using the Kollsman window correctly and setting the current altimeter setting within 100 NM of your location, the difference between a pressure altitude of 8,500 FT and your altitude is the difference in the calculation for the altimeter setting. At altimeter setting of 30.02 indicates you are flying a pressure altitude of 8,400 FT when the altimeter shows 8,500 FT.
The pressure altitude and the temperature at that altitude is what is required for the cruise calculations.