you overcome this by using a 1 1/2" ball valves on a 1" lines. from your overflow, which in effect adds zero back pressure to your return pump, allowing you to use the correct pump for your calculated water flow without having to sbtract for fittings, as well as water lost to the refuge on the return. less of a crap shoot when trying to mathc pump to overflow.
secondly you add a tee on your return with a ball valve that dumps back into your sump. if there is too much water moving to your tank for the overflow to handle (not a problem unless you buy more pump than your overflow is rated, say for future plumbing and what not) you open the ball valve and let some of the water pass through the second line and back into the sump. you lose effectiveness of your pump this way, but do not add unnecessary back pressure to it to try to slow it down. this second line also works great for water changes if you hook a hose onto it.
see broomer's diagrams for more detailed information on the return line to sump plumbing, as well as teeing off of the return with a large pump. he is a supersize kind of guy i think.
hip68, you are right on in your thinking btw as far as i am concerned. after 6 months fo research i settled on that design in february and have since seen 4 or 5 friends change their systems to match mine because they were not getting the export (growth) and numbers i was able to achieve. not to mention the greater circulation through the tank (10-25% loss when teeing off of return line, depending on how much you fuge once toy account for added head pressure).
this has been covered in a similar thread last week if you do a search for sump/fuge plans
HTH