fuel pressure dropping over time

In a perfect world the return line should be able to return ALL of the fuel back to the tank. Now, there are a lot of factors at play here. Most importantly the size and style of the fuel pump. An electric sized to flow enough fuel for 1000 hp will have drastically different requirements than a mechanical pump sized for 400hp. In @diymirage example his mechanical is sized about right for his combo and probably could get away with no return or regulator at all. Hell AJ said it’s what he uses, and his engine likely makes a little less power. The hardest the regulator and return will ever have to work is when the engine is idling so if your fuel pressure is stable at idle you don’t have a return problem.