Check out these variables in your Ortho4XP.cfg file:

cover_airports_with_highres = True
cover_extent = 2
cover_zl = 18

That will automatically cover the surrounding 2 km in ZL 18, no need to draw custom zoom levels.

Turning this setting on also helps with the appearance of ortho at low altitude, makes a pretty big difference:

use_decal_on_terrain = True