I would upgrade that gate valve you have on the Herbie drain. I think what you have there is called a knife valve, and I don't think you'll get the fine adjustment that you want from it.
I could be wrong, but most gate valves have round, screw-type handles like this. I know that on my system, an adjustment of almost immeasurable rotation causes an effect.
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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