[aerohead]

Since you have the book, I recommend that you re-visit page 129.
You're looking for the 'Effective Back-Light Angle' ( EBLA ), for notchback cars. which aerodynamicist, Adrian Gaylard, of Jaguar Land Rover shared with Julian, commenting that, ' Typically, for lowest drag, the EBLA is around 12 degrees.'
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I spent the weekend going through my resources, vetting Gaylard's rule-of-thumb, and I'll do a dedicated thread on it when I've finished.
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I believe that the origin of this EBLA is FIAT's SAE Paper 860212, which illustrates a schematic representation of a prismatic notchback car with a backlight angle of 22-degrees slope, with a rear deck, of which the trailing edge of the boot-to-rear trailing edge of the roof creates a 12-degree angle, and Cd 0.308, compared to a squareback of Cd 0.353, fastback of 33-degrees @ Cd 0.371, and fastback of 7.5-degrees @ Cd 0.327.
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The curious 'coincidences' that I'm witnessing is that, for the vehicles at, or near Gaylard's 12-degrees angle, they're also right at the most aggressive aerodynamic streamlining template (AST ) silhouette.
*BMW i8
*Ford Fusion 999 LSR
*Hyundai IONIQ
*Hyundai Prophecy concept
*Tesla Model X, with 3-position rear spoiler
*Tesla Model Y, with 4-inch carbon fiber rear spoiler
*Tesla Roadster-II
*Pontiac Firebird Tran Am
*Jaguar XE, with 47mm rear spoiler
*Porsche Taycan Turbo S, with 54mm rear spoiler
*Porsche 911 GT1
*Ford PROBE-V
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My thought is to get a hold of a 'blueprint' of your car, and do the AST comparison, and use this as your 1st-approximation. Do not use a 'photograph', only a bona fide technical drawing, which can depict the car in 'true length.'