Assuming the rule of 30 feet = 1 mb and no pressure change over the distance, the altimeter would read 300 feet.
If, as you say, the QNH at B is 998 there has therefore been a pressure drop over the flight of 12 mb (equivalent to 360 feet) allowing for the difference in height AMSL of both airfields.
Therefore, on arrival at B (having not twiddled with the altimeter knob on route) the altimeter would read 60 feet.
The pressure drop over the flight is 12 mb (equivalent to 360 feet). Airfield B is 300 feet lower than A (360-300 = 60).
I stand to be corrected and have opened myself up for some serious flak on this one
