I assume this has not been deployed - 50% or maybe 40% are fine I guess. To come up with good threshold values one would need to look at historic data for the past few months.
Nope, it is deployed (if by 'deployed' you mean DocTor is presently performing this check). From David's reply about moria1 it sounds like any check of this sort may be a red herring since they experiment with moria1, but I'll leave that up to you guys. Just let me know what kind of check you want.