A stock Exo Terra will reach (near) ambient humidity within hours or less after misting. It isn't possible to be very precise with top coverage recommendations, since your plantings, substrate, misting schedule, airflow in the room, etc, etc, will affect your setup.
Experimenting with covering varying amounts of the screen top with plastic wrap, acrylic scraps, or similar will give you an idea of how much you need to restrict ventilation.
If this is for your anole, I wouldn't get too worked up about hitting some certain target humidity. They tolerate any humidity above 50% as long as there is some fresh air coming in, and if there are brief excursions to lower humidity, they can hide in a damp spot until it is over. In my limited experience, dart frogs are nearly as tolerant.
Also if this is for your anole, you're going to need to leave a good portion screened, anyway, for UV penetration. I also don't like shooting heat lamps through glass, but I've read that other folks do that successfully.