The level of water in the fish tank will be at whatever elevation the inlet weir (overflow) on the internal box.  The secondary weir in the external box needs to be an inch or so lower then the inlet weir.  Both ends of the syphon need to be lower then the secondary weir.  For 200 gph, (total guess on my part) I'd try a 3/4 inch tube.  If it can't keep up move to a bigger tube, if air bubbles get trapped in the top of the syphon tube then move to a smaller tube.  If you have both problems then lower the secondary weir.

A picture is worth a thousand words, but I can't draw here.

I have figured out how I want the rear system of my overflow box to work, however I am still having problems determining some things.  One, how do I determine the level of the water in the fish tank?  Two, how do I make sure that I don't break the vacuum by pulling too much water out (vs the return flow) into the back overflow box?  What size siphon pipe should I use to achieve about 200, or a little more, gph? Are there any formulas I should use? Thanks,

Nate