It is really easy. They will give you QNH, which you will hear on the radio or ATIS as just ¨Altimeter Setting¨, for example they will say ¨Altimeter setting two niner niner two¨, so you will set 29.92¨ in the window. we don´t really refer to the Q code in the US.
Then, if you are flying at a place like KAPA in Colorado, the elevation is 5885ft according to the Airport Facility Directory (A/FD). on the same line, you will see ¨TPA¨ and it will tell you the altitude that you must fly and also the pattern height, so in this case it is ¨TPA 6885 (1000)¨. 1000ft above the ground, 6885ft MSL. so if you set your altimeter correctly, on landing, your altimeter will read 5885.
http://aeronav.faa.gov/pdfs/sw_195_10MAR2011.pdf
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you can also find good info at Airnav.com
Also, calculating pressure altitude is much easier using inches of mercury. since 29.92 is ISA, you subtract it from the local setting and multiply by 1000, and you get the correction factor.
so... (local setting-29.92) * 1000 + altitude = pressure altitude
clear as mud?