QNH
Hi Kraggy,
Just few things to be clear. If you set QNH at airport A, you can not read a height during a flight. Having QNH set, you will read altitude. If you depart an aerodrome A and fly with QNH set 990 mb(hPa) and want to fly at 500 ft AAL, it means you will fly at altitude 1000 ft (500 ft you fly above aerodrome A + aerodrome elevation of 500 ft). You will be higher for QNH difference (1010-990=20 mb= 20x28ft=560 ft, 1 mb =28 ft for precise calculation). With the same QNH of 990 ft within area of QNH of 1010 mb, it means you will be higher for that 560 ft (1000 ft altitude in the begining + 560 ft = 1560 ft altitude (AMSL). that means you will be above aerodrome B 1360 ft (1360+200=1560 ft).
Remeber this phrase "From high to low watch out below", what means if you enter area with lower QNH, you have to climb because you are lower than your altimeter reads. In your case it is oposite, you are higher.
I hope it helps,
Best regards