Example
I would suggest drawing while reading this...
(The calculations are not exact but normal "round" values are used to simplify)
Lets first assume some conditions: Airfield elevation 1000´
QFE: 993 HPa
Temperature: 34 degrees
Question one, calculate the QNH
27´/HPa (This is ISA and assumes 15degrees at MSL and 13,02 degrees at 1000´)
1000´
gives 1000/27=37 HPa
QNH: 993+37 = 1030HPa
QFF
The problem for calculation of QFF is that you have to use the real pressure lapserate which takes temperature into account. Normally equations are used for this but to show how it works i will use a simplified model which goes well with the rest of the pilot theory.
The pressure lapse rate changes about 4% for each 10 degrees off ISA temperature.
In this case the temperature is 20 degrees higher than ISA. (The mean ISA temp for 0-1000` is 14,01 degrees, in the isothermal calculation 34 degrees is used)
This means we correct the ISA lapse rate with 8%: 27*1,08=29,16´/HPa
1000/29,16=34 HPa
The QFF will be = 993+34= 1027 HPa
The difference in th calculations is simply what temperature are used for calculating the pressure lapse rate. In QNH ISA temp is used (in this case 14 degrees) for QFF the real temperature is used. (34 degrees)
Hope i made it clearer, there is always the risk of overcomplicating matters when discussing basics.