Thanks, that makes sense.
After researching this more, it sounds like there’s two ways in which a faulty fuel regulator can cause the fuel pressure to drop.
One is a weak or broken spring causing the regulator to not be able to adequately restrict the fuel flow, which would cause a lean condition. In which case you can provide the restriction yourself if there’s a rubber hose past the regulator. And I would imagine in this case removing the vacuum hose at idle wouldn’t affect the fuel pressure.
The other would be if the fuel is blowing past the diaphragm, which would cause a rich condition.