I have had a fuel pressure gauge on my 616 for two years now and also have replaced the lift pump with a new one. Gauge is between the filter and the IP.

I have a ball valve in the return line and have it closed, dead heading the Pump. (it is not recommended to do this)

My pressure readings dance between 18-20 psi at idle, if I rev the engine up it steadies out at 15 psi and during hard acceleration it drops to 10 to 12 psi.

At an idle if I open the valve the pressure goes to 0 psi and the idle drops!, so that tells me that the lift pump pressure affects the amount of fuel that is injected.

I would like to see my fuel pressure at 20 psi all the time with return fuel flow. I am ordering a spring that is better sized for the pressure requirement and speed of the cam.