If you really have to obsess about the fluid levels take a bright flashlight and shine the light through the side of the pitcher. I found that pitchers without prey in them typically have fluid levels of about 1/3 to 1/2 of the length of the pitcher. Near the coast the RH is generally between 30-60%.

By the time the pitcher starts to fill up with prey there's probably such a stew of rotting insects and commensal organisms that it doesn't need to fill to the brim with fluid.

They probably only fill about half way for a reason - the wind blows and a top heavy pitcher is more likely to flop over or break when tossed around.