Yes the formula is h=96T/P where h is foot per mb, T is Absolute Temp (deg C + 273) and P is pressure in mb.
However to work out QNE imagine you are on ground at an airport which is 1,000 ft amsl and the QNH is 963 mb so with 963 set the altimeter indicates 1,000 ft Now set 1013 - you are winding on 50 mb = 50 X 27.3 = 1365 ft The altimeter now reads 1,000+1365 = 2365 ft so this is QNE.
I stand to be corrected on this one but although the height change per mb changes with temp and pressure the change in indicated altitude will be the same for a given change in sub scale setting at different altitudes.