When I was painting my car it took me a long time to find these measurements, I was just surfing around today and this popped up.
The stripes are tapered in a Coke bottle like manner. If the stripes are just starting at the back and laying the tape to the front the stripes will have an hour glass in the middle look. By using this method they appear to be straight when looking over the car.
Hope this might help someone in need.