Basic OpenOffice Calc Sheet whith Barometric formula to calculate QFE from altitude
You can set up a flight plan for multiplayer missions where you have to put in the temperature, starting altitude and QFE and the altitude of your WP in the mission to calculate the QFE (you can leave the columns WP number and Coordinates blank). All informations needed are provided by DCS, temperature is found in mission description, the elevation of the WP can be seen in the F10 map.
The Barometric Formular can be found for example in Wikipedia.