O.K. Here's what you need to know. Fuel pressure is 3.5 bar(51.45 psi). As the voltage changes on the fuel pump so does the pressure. As pressure changes, so does flow. A pump flowing say 30 litres at 50 psi does not flow 60 litres at 100psi. As pressure goes up, flow rate goes down. Flow rates change and your air/fuel ratio changes. The ECU has no clue if your pressure changes. This is the reason for a pressure regulator. All FI systems have them. Current train of design is the fuel pump module(in tank) has the pressure regulator built in. This is known as a "dead head system". Only one fuel line is then required to head up to the engine. With aftermarket set-ups, you need to plan that the return line does not come back in where the pick up is. The fuel pump will be stronger than stock. You can have enough pressure from the return line to blow the fuel away from the pick up under high g-loads and low fuel. This doesn't happen with fuel tank modules cause the pick up is in a seperate cause inside the module. It's designed to stay full. The return will be in the outer case.
I'll pull my tanks off tonight and see if I can come up with any helpful suggestions. I'm thinking to mount it with the crossover tube in the front bottom being the draw. Get more usuable fuel that way. Return could go in and regulator be mounted where the stock pump was. That way you have one fuel line coming up the left side to the regulator. Return go right in the tank and not have another line running around. Stock line go right to the regulator at the stock pump location. Keeps the new fuel line short=less$$.
An adjustable pressure regulator would probably be a good thing for all these 990's running around with crappy fuel milage.