Honestly ... I love my prefilter box! I top off every other day or so and have no issues with it.
I don't see how you CAN'T top off if your water level drops low enough to not "spill" over the grooves of that box. From what I see in my tank, thats about an inch or so of water level your losing which could calculate into a 1/2 gallon or so. Depending on your tank size a 1/2 gallon could be a significant raise in your SG
Sometimes all it takes is to "tap" the pre-filter box down just a hair and your levels will raise enough to fill that chamber back up.
as for the consistancy of your skim mate the skimmer is producing, Fluff was spot on ... you have to move the collection cup up using the O-ring, the higher your cup the drier your skim. the lower the cup the wetter the skim!
Constant tinkering is a MUST in the saltwater hobby. If you don't, your bound to have issues somewhere down the line. JMO!
WOW ... my first post here in a LOOOOOONG time!!!