[aerohead]

Forgive me for butting in but I wanted to toss out something that might help you answer your own question. Aero isn't that mysterious. Your truck is submerged in air,and as you advance forward,you displace a volume of air equal to your frontal area,times your distance.Air has mass(so momentum and inertia are involved) and air is viscous(so there's friction when you move through it).---------------

With the Tacoma,your trick is to allow the air to recover to the position it occupied before you came along with the minimum amount of separated flow. Once the air has reached the point of maximum cross-sectional area ( about where your head is, outside around the cab),you have converted atmospheric pressure into kinetic energy. The whole point of streamlining,is to provide a ramp which is gentle enough,such that the air can rebound(like a spring),without leaving the surface of the aftbody of the truck. Any reduction in the cross-sectional area of the truck without flow separation will produce a like reduction in drag. If you can reduce the wake of the truck 10% without separation,then you reduce your drag by 10%. If you imagine the imaginary teardrop tail,extending rearwards behind your cab,divided into 10-sections,each section with 10% less area (turbulence-free),the further back you go,the less drag you have,losing 10% for each "section." And you chop it off where ever you choose.That's all there is to the Kamm-back! Kamm and others did recommend that you chop it off when the (clean) cross-sectional area is reduced to 50% of frontal area. -------------

You can cut it anywhere you want and you can calculate your drag reduction as a direct function of the area of the remaining wake. That's all there is to profile drag! But remember,the curvature of your structure cannot exceed the curvature found with the teardrop or separation will occur,and you're right back where you began.Careful examination of your roof will tell you if Toyota gave you a little curvature to begin with,you can take it from there.