Ie; Aiefield elevation of 60ft AMSL ÷ 30 feet = 20hpa or mb? So you add 20mb onto the subscale or add 20ft to the altimeter?
60 divided by 30 is 2.
2hPa is added to the QFE to convert to QNH
2hPa is subtracted from the QNH to convert to QFE
2mb is the same thing as 2hPa.
If you meant to say that the airfield elevation is 600ft, then the conversion would be 20hPa (or 20mb).
Because air is compressible, and the atmosphere is approximately 100km thick, the air at sea-level is compressed by the huge weight of air above it. The air at 10km altitude has a lighter weight of air above it; the air at 20 km even less; and so on. This variation of compression with altitude means that the reduction of atmospheric pressure with altitude is not linear.
At lower levels, the pressure reduces by about 1hPa for every 27ft.
At 40,000ft it takes about 70ft for the pressure to reduce by 1hPa.
So, for most airfields, a rate of 1hPa per 30ft is accurate enough.
The slow rate of change at high altitude is the reason that RVSM levels cannot be used above FL410. Pressure altimeters (that use the change of atmospheric pressure to indicate altitude) are not accurate enough at these higher levels to ensure that 1000ft separation is sufficient.