[aerohead]

kafer,if you will GOOGLE maxmpg ,go to that site, join if you are not a member,and then go to the PHOTOS section,under MISC. I've got 222 aero photos over there.Photo HPIM0248 shows Mair's boattail research graphics,and HPIM0222 shows an ideal aero form with proper boattailed back.Your Jetta begins it's body taper close to the B-pillar I believe,for both the roofline,and body sides.If you have photographs of your car you could photo=copy that onto a larger sheet of paper,mark your theoretical terminus at ground level ( approximately 4 body heights behind the point where the body begins to taper(,then take a French-curve to draft the elliptical line from the Jetta's roofline to the terminus.Blow this image up as much as you like to take scale measurements of,for the layout of your project.Once you have a few data points in full-scale,you can use a 20-foot length of 1-inch,schedule-40 PVC pipe to use as a huge French-curve,to establish your full-scale layout.Should the projected lines from the sides differ from that of the roof,you'll have to integrate that into the design.As already mentioned by other members,performance of the tail will vary with length.Weigh out conflicting parameters and compromise at some arbitrary length.Safety is PARAMOUNT! Since you will effectively be attaching a weather-vane to your car,you must honor what has been established empirically to be safe,mainly,generous upper corner radii,which increase in radius,as the tail projects rearward.These curved corners will help local pressure differentials to bleed off and frustrate potential yawing moments,notorius in early long-tail cars.Your center of pressure should end up behind your CG and at speed the car should exhibit over-steer if anything,with which most drivers are comfortable.Don't go crazy with length,as you may start knocking into stuff when rounding corners.