True elevation with RTK-Drone

Hey,

i‘m using an rtk drone (dji phantom 4 rtk). The telemetric **is recording the ellipsoid hight from wgs 84.
I know the exact shift betwen this height an the real hight above sealevel from controll points.

What is the best way to change the hights for LAS an pootree export?

Is it a linear offset?