[freebeard]

So a 10% extension, assuming a perfect execution, etc, can return a 10% reduction in drag?!

Yes, but not total drag reduction. Just the reduction that would obtain if it was extended completely. And the shape is completely dependent on the 30% of forebody. Here's aerohead explaining:

*The streamline body which the 'Template' is based upon has Cd 0.04
*When the body is brought into 'ground reflexion'(Rumpler/Prandtl) it jumps to Cd 0.08.
*When you cut away for ground clearance,you get a car body of Cd 0.08.
*When you add wheels and tires the drag jumps to Cd 0.12-13.
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*The 2.5:1 streamline body of the 'Template' has zero separation
* As a car body,the Template' will have no separation.
*If you make the aft-body steeper,it violates W.A.Mair's research on aft-bodies,which found 22-degrees to be the max angle air will follow without separation and vorticity.( Hucho allows 23-degrees ) (I'm conservative @22-degrees).

http://ecomodder.com/forum/showthread.php/biggest-gains-back-19906-4.html#post280363
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I found a quote where aerohead explains the Template in general:

5:1
The 5:1 L/D ratio is for the 'apparent' ground reflection body,measured at the ground plane.Along with it's sister image below the ground it forms the 2.5:1 streamline body.
A car and it's reflection below the ground is the premise for evaluating it's aerodynamics.
The concept was first realized and introduced by Paul Jaray and gained traction after Ludwig Prandtl's and Edmund Rumpler's wind tunnel work.
The original 'Template'-C illustrates this concept.
Paraphrasing Hucho:" the low drag body is represented by the streamline body and it's reflection below the road surface."
I have cut away the ground clearance of the body based upon SAE 'approach,ramp,and breakover clearance angles.
If you extrapolate the aft-body out and down to the ground plane,it creates the aft-body of the 2.5:1 streamline body,never exceeding Mair's 22-degree angle.
I've essentially ignored the forebody,as mentioned in the 'Template' thread,as Hucho recommends we concentrate on the back of the vehicle for streamlining purposes.

http://ecomodder.com/forum/showthread.php/honda-insight-tail-extension-project-13533-47.html#post319931

It sounds like you looked closely at A-F, because you see questions. Notice that F is the only one with plan taper and the mess that E is in comparison.

Also, aerohead mentioned Mair; so here's a look at his work:



Relative perfection starts at l/d of 0.5; that's how sharp the curvature is. Once again truncations are possible, so that's two ways of accommodating necessity.