U can calculate the QFE on any spot of the map as long as u know your current elevation and the target elevation, assuming that the pressure remains constant over the whole map.
Lets say your current elevation is 500 ft.
If u set the altimeter to zero, then you know your current QFE, lets say its 995 hpa.
Now lets say the target altitude is 2000 ft.
2000-500 = 1500, thats the difference in altitude between you and the target.
On a standard day 1 hpa = 27 ft, so 1500 / 27 = 55.5
Since pressure decreases with increasing altitude, you need to subtract 55.5 from 995 = 939.5
That is the target QFE.