Yes, that humidity can be a little touchy. With the reservoirs on the 1588, you can try filling different ones, one at a time to see what happens. I generally just go with the center one like they say and it works pretty well, but sometimes doing just that one I can get various readings, from low 40's to mid 50's.
As was mentioned, surface area is what counts, not depth of water in the reservoir. If you can't get it down ay other way, you can put aluminum foil or something similar over part of the reservoir to lower the surface area.
Something else to watch. It is hard to see how much water is in the reservoirs. If you overfill the reservoir and some runs into a neighboring reservoir, the surface area will go high for a while, until the excess dries up. If your humidities are jumping around, that might be a cause. I don't think you need to ask me how I know about that one.