>It's failrly straight forward. The altimeter if set to QNH will give you your height above sea level, so If you know the height of the airfield below, you simply take the airfield's height away from your altitude. Unless you call the airfield up and ask for the QFE, that's the only way you do it.<
When you are flying on QNH you are at an ALTITUDE (above msl) and when on QFE at a HEIGHT (above aerodrome/touchdown level). Also it's the airfield ELEVATION (ie amsl) rather than "height".
As correct altimetry is so critical to safe flight it is important that the correct phraseology is used and understood.
As has been said above the QFE/QNH arguement has been around since aviation began. There are pros and cons on both sides. However try operating out of places like Nairobi (circa 5,000 ft amsl) or Mexico (circa 8,000 ft amsl) using QFE! You have lots of winding to do to set QFE for approach and/or QNH on Go Around.
Having witnessed many Go Arounds in the back of the Sim I can tell you that when we used QFE there were far more altitude busts compared to when we changed to QNH operation.