How long to be effective??? Look at the Lexus RX350 for inspiration, the Kamm spoiler on it is 8" back max and maybe 3" from the window surface. The "Angle" NEEDS TO FOLLOW THE ROOF! Just extend the roof. If you drop the kamm angle, you'll create lift, if you go up with it, you'll create downforce drag. Place a long flexible strip of wood on your roof, holding it in the center of the roof and the strip extending back over your rear window, the Kamm should be built to match the curve created when you push down on the strip of wood over your hatch and it just makes contact with the back part of your roof.
Don't get caught up into thinking you need to cover the whole rear hatch or extend the Kamm even much beyond even 1 foot. The air simply needs to make a clean departure from your roof to do some good.
