I can confidently now say that my 42' Californian has an 18.5' air draft. The place where I had my boat hauled out has an 18.5' clearance on their door, and we slid under with barely an inch to spare.
Since we had 2' removed previously, I'd guess that a non-modified boat has about a 20.5' air draft.
Hopefully this help others who are seeking this info...