Ah, my configuration is alloy cover, fairly large hose to air/oil separator, which has a PCV to an intake common plenum. Separator drains to the sump. Even so, without the plate, I still got oil coming out of the cap. The plate stopped that.
Regardless, assuming the pressure inside the rocker cover is the same as the crankcase, you would need a lower absolute pressure in the intake at sufficient flow to overcome the total blow by such an amount that ambient pressure is higher than the resultant crankcase pressure to insure flow is into the engine from that breather hole. At wide open t