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[fkshiu]

Simply have your return line exit at the water's surface to minimize back flow.

All you need is a rough estimate of back flow volume so just use LxW of the bowfront at its widest point (so you'll be sure that actual volume is less) multiplied by the half inch or so that will backflow before the siphon is stopped.

[hesham]

To calculate the volume of the section of your bowfront aquarium volume expected to overflow, suggest using the aquarium volume calculator at;

http://reef.diesyst.com/volcalc/volcalc.html

This is a great calculator site if you are looking for accurate volume data for your bowfront.

Take the depth of your overflow teeth and enter as depth in the calculator along with the other dimensions of your bowfront. It will tell you the volume which you then can use as the additional volume your sump needs to handle above the standard sump running volume.

Now this will be a little conservative as only a thin sheet of water should go over the overflow teeth.

Watch out for any siphon effects with your return plumbing as this can empty your tank to the level of the return nozzles. Sorry if this last point is too obvious.