Snowpetrel a reply from Baeckmo from Boat design forums also said;
You have two issues here:
A/ The tank end of the return should always be submerged, particularly if the tank is lower than the engine. If open to air, the line may slowly be drained when there is no flow. Finally, this can lead to the draining of the injection pump itself, with starting problems as a result.
B/ Depending on the actual design of the injection system, the return flow may, or may not be a considerable share of the total flow. Now if the return leads back to the main tank, you can reduce the operating time on the day tank, since it will be quickly emptied by the return flow to the main tank.
I would take the return to the day tank and make certain that the end always is submerged. The idea with a day tank is to isolate the main supply from the operationally necessary volume for safety
So I plan on taking the fuel return to the base of my 30 ltr day tank as it is above the engine.