In a properly designed system you don't need any flow control to prevent overflows when the power cuts out.
You should use an overflow box, not a straight siphon. Balancing the siphon flowrate to the pump flowrate is difficult and provides a lot of opportunity for flooding.
Your overflow should be capable of a higher flowrate than your return pump. You want the pump to be the choke in the system.
With the pumps turned off, fill the tank and the sump. If you don't add any water and your system is set up correctly, you shouldn't have any flooding when you turn the pumps on or off.