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Although I can't find the post/cite/reference, the results of some OEM aero-testing showed that the efficiency of the wheel dams, especially the front ones, is highly sensitive to both location and size… and a real PITA to get right when DIY-ing.
A suggestion: where your dam is going to be located is pretty much a function of the distance from the tire tread to the wheel arch + a little.
At this point, try to piggy-back on somebody elses work.
Measure the distance wheel-wheel arch + distance on you car. With that in hand, go looking for another car with an OEM dam at the same distance. Measure that one for size, and note the sideways, in-out location. The rear dam is typically nearly centered on the tread width. Front dams are typically displaced quite a bit towards the centerline of the car apparently to reflect that the airflow is moving outwards at an angle. Also they are not as wide as the tread.
FWIW, almost all small cars from Europe and Asia have these dams. For the most part only the most recent small US cars do.
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