With PL filters what I do is aim my drone at the horizon while on the ground and with the live camera view on your device, hold the PL filter just over the camera lens and rotate it while watching the impact it has on the live feed. As you rotate you will see the darkening and lightening at different positions and then install it in the orientation that looks best. I have made a little mark on mine so I know basically what orientation it is in without a lot of trial and error. It's amazing how much good filters change the quality of your final product though!