This is rough but works for me: Add 10% to the height above the altimeter source, this works for - 20 celcius for DH, DA, MDA. Pay close attention to additive values in mountainous or obstacle rich terrain, for these, use the 10% rule and round up to the next 100' increment on procedure turns, fix crossings, and arcs etc. The greater the height above aerodrome and the cooler the temperature at the altimeter reporting station, the greater the error.