If you want to derive QNH from QFE, a quick way is to divide the airfield elevation by 30 and add that to the QFE.
Similarly, you can derive a QFE by subtracting 1hPa from the QNH for every 30ft of airfield elevation.
I've been googling this question and found your post, however could you give me a example?
Ie; Aiefield elevation of 60ft AMSL ÷ 30 feet = 20hpa or mb? So you add 20mb onto the subscale or add 20ft to the altimeter?