One more thought. Another reason why I think it makes sense to think in terms of the joint not being a corrugation that is counted, is to think of the panels independently. That is, the top panel of a door, whether the joint is raised or flat, has x number of corrugations. Until it is actually attached to another panel, the bottom raised portion of the panel wouldn't be a corrugation, it would just be the bottom frame of the panel. 

In other words, if you were constructing a door, and the current panels had a raised joint, you'd select two 5 corrugation panels and one 4 corrugation panel for door #3. If that makes sense.

Randy
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Randy Hammill
Modeling the New Haven Railroad 1946-1954
http://newbritainstation.com