With the vacuum hose on the stock regulator diconnected, your minimum fuel pressure should be 47-50 psi. The adjustable FMU should only raise fuel pressure from that point. If you leave your stock regulator vacuum hose connected, it will vary your minimum fuel pressure from 38 to 47-50 psi depending on how much vacuum it sees. Then the FMU would be trying to vary the pressure at the same time. Save yourself some tuning headaches and disconnect the vacuum hose to the stock regulator. If you're really concerned about the stock regulator. Take it off and examine it. Maybe you can tear its guts out and keep it from regulating anything . Its only held on with a snap-ring .
