Ohhhh ok I gotcha. So let me ask this.. If the power goes out and the pump stops, what stops the water from draining into the sump? Or is this where you factor in enough room in the sump to accommodate this water?
Just like what you said. Your sump should have enough room in it to hold all the water from your plumbing and from the lowest point in your DT which will either be your lowest overflow or the lowest return (since the return will create a back siphon)
I used 1/4" which should be good for pretty much any standard sized tank conversion. The baffles have pressure on both sides by the water so it cancels itself out unlike building a tank where one side has a ton of pressure while the other side none.