Ymmv but at around 92%-94% you can bet on a blank line in most bases. BWI & MDW are closer to the 92% end since they are commuter heavy. With the overmanning in Jan/Feb I'd wager that local guys will bid reserve in the hopes of sitting at home. At 90% I'd say you are all but assured of holding a line in any base.

I don't think bidding 70% out west would be worth the potential pain you'd be enduring in order to commute there.