I too disagree with the notion that max AoA is best. There are two parts to this question. The first one is, what do I want out of the airfoil. Do I want maximum downforce, maximum efficiency (best trade-off between drag and downforce), lowest drag ... et cetera). The answers to these questions can be found in the performance curves for the particular airfoil involved. You can also put wool tufts on the airfoil and experimentally verify the AoA at which the airfoil stalls, though that won't help with determining the other points.
The second part of this question is, what AoA makes my car fastest. Obviously, experimentation is the only way to find this out. However, the answers to the above questions may assist this process, for example a very slow, twisty circuit you might want to start with maximum downforce, decreasing the AoA to see if lap times drop, and if they do continuing to decrease it until they stop dropping. Similarly, on a fast circuit you might start with the low drag setting, and progressively increase AoA until lap time stop dropping. If you have the performance curves for your airfoil, you will be aware that it continues to produce downforce even at positive angles of AoA, something that is not intuitively obvious to the layperson.
This of course totally ignores the critical question of car balance. Unless you are running totally free aero, or near enough to it (looking at you, time attack) front end aero is the limiting factor. As a result, it may be necessary to run sub optimal rear aero to minimise understeer and protect the front tires, or to keep the backend under control.
Regarding the comment above about endplate efficiency, some of you will be aware that I lost a Singular endplate at SMP. This happened because the endplate levered the bonded-in mounts out of the core of the wing, a result of the differential air pressure. Suffice it to say the replacement mounts are quite different. So for those who are playing at home, don't underestimate these forces.
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