I agree with Matt on the feeding ring. Without it, my flake immediately got sucked into the overflow!
I used flake food with a rotating penn-plax feeder. it worked, but using it for over a week seems to fail because the flake gets all clumped up by the moisture! Crushing the flake seems to make the feeder work a bit better.
My final advice is to TEST and observe the feeder for a few days before relying on it to feed your fish. Then start it up a couple of days before you go on vacation to ensure it works.
As a backup, I always toss in one or two of those "7 day pyramid feeders" which are those dissolving blocks.
Oh, and clean your skimmer before you go to make sure it will run at optimal efficiency in case of a vacation-time disaster, and also to prevent it from overflowing.
And what about top-off? Make sure your reservoir is full. What I do is add extra RO water to the sump so that it can absorb some un-attended evaporation without pushing the salinity too high.
V