I don't think Amateur Turbines was asking about density altitude, but rather the correction required for true altitude.
The quick and dirty formula for that is that your true altitude (strictly, height above datum) is 0.35% lower/higher than your indicated altitude per degree C that the atmospheric temperature is lower/higher than ISA.
e.g. for ISA -10 degC, an indicated altitude of 10350 ft is actually 10000 ft (if the altimeter setting is correct at sea level).
Another way of putting that is 3.5 ft/degC/1000ft. If the difference between that an static's 4 ft/degC/1000ft makes a difference to your health, you're not doing a very good job of height keeping. <img src="smile.gif" border="0">