It's best to treat at skin level, at least in a few locations. We've done it both ways, spraying up at the roosting birds from underneath, and then top lines. See how effective your first treatment is, by looking at them at night with that flashlight in a few days.
Here the mites come in on...
At night, while they are roosting, and you with a small flashlight or headlamp, it's much easier to spray this birds!
And you need to wear long sleeves. pants, and at least a N95 face mask, so you aren't inhaling anything out there.
Mary