Gross error method:
WINDING ON MILLIBARS = WINDING ON HEIGHT
In other words, if you're increasing the value in the subscale setting window by 1mb then you'll be increasing the indicated value (ie. height/altitude) by about 30ft.
Best way to tackle questions about QFE/QNH/1013mb is to start by visualising your aeroplane with the correct QNH set.
In CONC757’s first example the altimeter was reading minus 31ft with a QNH of 1029mb set. To display QFE (the altimeter would indicate zero feet with the aircraft on the ground) you have to ‘wind on’ one millibar which means the value in the subscale pressure window (or QFE as it is now) is 1030mb.
In CONC757’s second example approaching an aerodrome at an altitude of 1230ft with QNH of 1015mb set and you then set a QFE of 987 millibars for landing. In this case you have ‘wound off’ 28mb (1015mb minus 987mb) worth of height which results in the altimeter indicating 840ft (ie. 28 x 30ft) less than it did before. The altimeter would now be indicating 390ft (ie. 1230ft minus 840ft).
In the CAA questions, there’s always an answer for someone who has correctly worked out the value but applied it the wrong way round (ala Pitts2112 and Cusco).