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.