Re: How much passive ventilation is too much for plants?
I have had an 12" * 12" * 18" Exo Terra, so I think I am able to picture what you are saying. (By the way, is that the way the dimensions are always ordered? Width * Depth * Height, and not another arrangement of those dimensions?)
IIRC, the screen top of a 12 * 12 * 18 is divided in the middle by a plastic line that is horizontal, when viewing it from the front of the tank.
If I am correct, the space I would have reserved for a screen panel would be somewhere between the two settings you described in the first paragraph --- although when factoring in the space covered just by the black plastic lines of the stock ventilation panel, I can't say which of the two of those my hypothetical 4.5" * 24" screen panel would more nearly resemble. Not that it matters, though; being anywhere in the window between "healthy drying-out" and "pretty moist" is perfectly fine.