Also this an excerpt from a aero study that I think I found on this very forum,it talks about how the designers of the Golf went about the hatch angle.
"The fact that two completely different types of flow occur depending upon the inclination of the rear end, which lead to different drag, was observed for the first time during the development of VW Golf I. Janseen and Hucho varied the angle of inclination of the rear end <p (see Fig 5.14) in increments. The drag coefficient is shown versus the angle of
inclination <p in Fig 5.14.
Fig 5.14 Influence of rear end slope angle <p on drag coefficient, separation line and wake, measured on VW Golf I (Rabbit)
The development work in the wind tunnel began with a high angle of inclination at the rear end, <p = 45o. At this angle, flow separated at the end of the roof. The drag coefficient was
0.40. As the angle of inclination was diminished, the drag suddenly increased by 10 percent at <p = 30o. The line of separation jumped down to the lower edge of the inclined rear end. Two strong inward-rotating longitudinal vortices were observed which induced very low pressure on the slanted part of the back. As the angle was reduced further, the drag dropped again. At <p = 15o, a very flat angle, a drag minimum resulted. At still smaller angles the same flow forms were reached as with a square back: CD = 0.40. In the area 28o< <p <32o
a bistable condition was observed. Depending upon the curvature of the rear edge of the roof, separation occurred at the top or bottom of the inclined rear end."