optimum crankcase vacuum

[SeptemberSquall]

i suppose only a few mm/Hg is completely satisfactory?
surely too much will start sucking in the sump gasket and etc.

[YelloRolla]

up to 15" in a dry sump set up. I imagine that a stocko that is using manifold vacuum is going to be lot less.
Those people who are venting to atmosphere via an oil/air separator are going to have no crank case vacuum, and in a stock set up, naturally aspirated engine, there will be no vacuum at wide open throttle. So I am thinking "optimal" is subjective.