When calculating the pressure altitude for aircraft performance I learned to calculate it the following way:
Pressure Altitude = (29.92" - altimeter setting) * 1000' + elevation
So at an airport at 3000' elevation and an altimeter setting of 29.92" I can expect the aircraft performance to be that for 3000'. I never questioned that but now I wonder why don't we just care about the altimeter setting? If the altimeter says the station pressure is 29.92" sea level, but I am at 3000', then the airplane still thinks it's at sea level when it comes to performance. Is that wrong?
Same for an airport at 3000' elevation and an altimeter setting of 28.42" I can expect the aircraft performance to be that for 4500'. Again, why not 1500'? The airplane should think that it is, regarding pressure, 1500' above sea level.
I think that would make sense for en-route performance. If the station pressure along the way is 29.92" and I fly above at 4000', my pressure altitude will be 4000' (0*1000 + 4000). If my station pressure however is lower, say 28.92" for instance, then the pressure altitude above at 4000' would be 5000' ([29.92"-28.92"]*1000 + 4000).