Of what importance is it? All the
fuel the engine doesn't need goes back.
As long as you valving system ensures that the overflow goes back to the same tank it came from, no problem. Some systems (mine is one of them) has two valves and if forgetful it is possible to return
fuel to another tank and that's not clever, especially if that tank is already full.
If you have a smaller volume "day tank", use it always because returning
fuel to a tank where it will live for a while will lift the temperature of that tank and promote condensation and the growth of bugs and other uninvited guests.
Andre