Barometer correction - BARO_SENS_QNH proposal

When digging for wrong value of ALTITUDE_AMSL i have arrived to adjust parameter BARO_SENS_QNH, which is the pressure correction on my location.
Pls correct me if i’m wrong - in case of too big VDOP estimator will take into calculation the altitude from barometer.
And finally if it is a good idea to calculate this parameter using GPS altitude once when we have good VDOP at startup of flight controller?