How to calculate aerodrome elevation given QNH and QFE
QNH is the pressure setting that will show the aerodrome's true altitude at field level, QFE is the pressure setting where the altimeter will show zero feet at field level. The rule of thumb is that pressure changes 1 hpa per 30 feet, so the formula you would use is (QNH - QFE) * 30feet/hpa = field height. In this case that is (1031hpa - 985hpa) * 30feet/hpa = 46hpa * 30feet/hpa = 1380ft. 

If the QFE is greater than QNH you would get a negative number, which would mean the airfield is below sea level. This is pretty rare, but they do exist, so don't mess with the QNH - QFE order.