I had a -8 feed and -6 return (which worked fine for an EFI application) and could not get the fuel pressure below 12 psi. Ended up putting a dead head reg between the return reg and the carb to knock it to 6psi. Fuel still circulates and keeps the pump happy.

It's a bandaid, but it works fine and was cheaper than buying all new line and fittings for the return line.