In comparison to manufacture's specs are you using minus development, stand...?
Are you "zoning"? If so details?
Spot meter or incident?
Are you using old Petzvals or newish multicoated lenses?
Is part of that rating a safety factor?
My intent here is not to put you on the spot c6h6o3, but to help marciofs understand how you made your choice and at least some of considerations that go into the decision.