[aerohead]

Years ago, I graphically analyzed Hucho's 2nd-Edition Figure 4.59, which comes from SAE Paper 810185, by Buchheim et al..
I've done a linear interpolation between the 27% aft-body and 36% aft-body, tested and reported in Figure 4.59.
An 18-degree drag minimum aft-body rear slope angle occurs, approximately, with a 32% aft-body, @ Cd 0.436.
Should a spoiler be added to the contour:
* the wake area would be increased
* flow separating at the top of the added spoiler, would be more horizontal, directed 'away' from the wake
* the flow would be at a higher velocity
* the flow would be at a lower static pressure
* communicating this lower pressure into the wake could lower base pressure
* increasing pressure drag
* increasing overall drag
* at normal posted speed limits, any reduced rear lift might be statistically meaningless
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The 1969 Dodge Charger Daytona, with a 41.7% aft-body, exhibited its lowest drag with the roof rolling into a backlight angle of 18-degrees. The presence of the reflexed boot ( trunklid ) created a departure angle of 15.5-degrees, for an indicated Cd 0.270.
The same roof, rolling into a constant 12-degee backlight and 'fastback' came in at Cd 0.275. The trailing edge of the 'fastback' actually delivers the flow 'above' the streamlined pathway, both increasing wake area, plus re-accelerating the flow.