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However, when the belly pan was attached to the bottom of the air dam, that was when the magic happened. I suspect that this configuration allowed the cooling air being pushed/pulled into the engine compartment filling the vacuum behind the air dam, greatly reducing it drag.
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The first problem with the 'rule of thumb' is that the air won't move straight back from the bottom edge, as you say.
Secondly, lowest point is ambiguous. The tow hook on my Superbeetle is 1.5" lower than anything else. it's frontal area is miniscule.
A while back I tried to get 'first approximation substituted for 'rule of thumb' but it was like King Canute ordering back the tide. [We jest]