Do I need a valve on sump plumbing?????

[Blom]

Well to state that the equation isn't useful here I believe is wrong. You are correct that it depends on a few things, such as air water mixtures, whether or not it is laminar/turbulent flow. You could easily solve for a Reynolds number and jump into a whole bunch of fluid mechanics equations. But the RC calculator would of course be the fastest and simplest way to go. Good find.