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A good way to start might be by taping a few tell-tales on the rear window, to see if you do happen to have attached flow there now. If so, an opaque tail extension is relatively easy and worthwhile. The classic 1st guess angles are 15 top and sides, and 4 on the bottom, but there are a lot of considerations that can raise or lower those estimates. If you do have attached flow on the rear window now, it may depend on the high pressure zone ahead of the spoiler, so you can't just continue the same line with confidence.
Avoiding lift usually costs a bit in drag, and is of importance depending on weight, speed, and wind, etc.
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