The way the FMU works is by blocking the return line and spiking the fuel pressure. If you were to install it on the feed line, you'd lose almost all of your fuel pressure when the car hit boost. FWIW, a FMU is not the ideal way to go.
Yea its like clamping off a line and that makes the pressure rise....which is what it does in the fuel rails to make up for the smaller injectors....not the Professional and most accurate way but it will get you going. To do it properly and make the most power is to get rid of an fmu, put in bigger proper size injectors and get it dyno tuned.