where did performance calcs come into a discussion of pressure altitude?
I'm coming around to the view this is a wind up. But just on the off-chance you really didn't know, bose-x, density altitude is pressure altitude corrected for non-standard temperature. Oh, BTW, density altitude is what we use in performance calculations ... so performance calculations require the input of pressure altitude ... oh, WTF!
I really have reached the end of the road on this one. Just to summarise for Dave, though, he has apparently concluded that there are four potentially appropriate altimeter settings. Using the example of his original question (where we are told the airfield elevation is 968ft, the QFE is 992mb and the pressure lapse rate equates to 30ft/mb), when he sits in his aeroplane doing his pre-take-off checks he can:
a) set the altimeter to 992mb, the QFE, wherein it will show the height above airfield elevation and read 0ft;
b) set the altimeter to 1013mb, the standard altimeter setting, wherein it will show the pressure altitude (whilst on the ground, this is the aerodrome pressure altitude or QNE) and read 630ft;
c) set the altimeter to 1024mb, the QNH, where it will show the altitude above sea level and read 968ft; or
d) set the altimeter to 1045mb, the noise attenuating pressure setting, where it will show the altitude above an arbitrary and irrelevant reference and read 1,598ft (since this altimeter pressure setting is not defined in aviation, I've labelled it after a well-known headset!).