How Much Vacuum??

Yes, for a test, plug the hose, and see where the vacuum goes to. If it is a stock unit, then it will be wide open at that vacuum level, thinking you are at heavy cruise mode with that level of vacuum. (Well, it doesn't really THINK LOL....) PCV's are designed to shut down to a lower flow level when the vacuum rises above a certain level, so that it does not interfere with idle operation. I have found that the stock PCv's open up to cruise flow at around 14-15" of manifold vacuum so that is the reason to think this is wide open right now at your low idle vacuum.

I really would expect several inches more idle vacuum; my son's 340 runs 14-15" at 850 RPM with a Crane Z268 cam. It has a wider LSA so I would expect yours to be a bit lower.