I got tired of removing the skimmer cup every time it got full....so I put this little modification on yesterday. A little 1/4 inch line with a valve on the end so that whenever the cup looks a little full, I can drain it without shutting down the skimmer.
I'll put one other bulkhead in as an emergency overflow. Emergency overflow will just fall back into the tank. If the cup gets too full, the skimmer starts percolating like a coffee maker.

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400 gal reef. Established April, 2007. 3 Sequence Dart, RM12-4 skimmer, 2 x OM4Ways, Yellow Tang, Maroon Clown (pair), Blonde Naso Tang, Vlamingi Tang, Foxface Rabbit, Unicorn Tang, 2 Pakistani Butterflies and a few coral gobies
My Tank:
http://www.canreef.com/vbulletin/showthread.php?t=28436