AIUI, you take your height above the airfield whose QNH or QFE you are using.
The QFE is the pressure at the airfield - when you're on the airfield, the pressure will equal the QFE, and the altimeter will read zero. As you gain height, the pressure will drop off slower than ISA due to the low temperature, so you need to add an amount to the indicated altitude proportional to the height you are above the airfield.
Likewise, QNH works in exactly the same way. It is the pressure at sea-level, corrected for the ambient temperature such that the altimeter will read airfield elevation when on the airfield. Therefore, it is the height above the airfield that determines how much correction you need to apply to the indicated altitude.
I think.
FFF
--------------