Need advice on extending out a segmented boattail ( I have no idea how to word that )

[aerohead]

I read through a few pages, and finally found Darin's road test data.
He WAS testing at 50-mph. Average, 39.5-40-mph.
In Austin, on the Toll Road to San Antonio, at the posted speed limit, we'd experience an aerodynamic power requirement, 959% higher than the Canada numbers. Rolling resistance would only climb by 212%. This is WHERE a boat-tail would tell on itself or not, and be an interesting comparison.
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The rule for boat-tails is that, if you reduce the wake by half, you've automatically reduced drag by half.