Syphon Effect Calculations

I've never seen any calculations for this, just hand-me-down specs or drawings that show a small hole (say 3/8" or whatever size). However, there are many possible situations, so probably no simple answer. You could estimate how fast air or vapor might be drawn through the hole, calculsate the orifice pressure differential, and compare to how high liquid must rise to go over the siphon. (That may seem vague, but I'm just trying to cover the general situation.)

If the Overflow pipe is located at the level where it should drain the extra fluids, and it has no internal bends (downward into the fluid), there will be no syphon effect when the fluid level reaches the pipe orifice.

Your problem must be to have the correct pipe size(diameter) to accomodate the event of overflow against the incoming supply: Flow rate through the pipe must be higher than the expected flow rate coming into the tank.

The vent pipe in this case is to avoid trapping air in the overflow pipe preventing an easy drainage: Put a vent pipe of 1/2 the diam of the overflow pipe when sized properly.