Testing can be very easy and cheap.
I've previously measured the downforce created by the rear wing on my Insight by measuring suspension deflection. But, because the wing is mounted on the rear hatch, I thought I'd try an even simpler method.
I placed some springs on the rubber bump mounts of the hatch...
..and then placed a vertical, marked piece of wood inside the car, near the vertical part of the hatch. I placed a piece of tape on the rear glass, aligning it at the zero mark.
The vertical scale and mark were visible in the rear vision mirror.
I then drove the car, altering the wing angle against a bubble level scale for each test....
...until I could see in the rear vision mirror when the wing caused the maximum compression of the hatch on the springs.
Back home, I then added weights to the wing until the compression was the same as the maximum recorded on the road.
So an easy way of measuring:
(1) best wing angle for maximum downforce
(2) the actual amount of of downforce created by the wing
And on the Honda?
(1) 0 to minus 5 degrees reference angle
(2) 8kg downforce at 100 km/h (18lb at 62mph)
Off topic addition:
Once you have real, measured data, you can then apply theory with some confidence.
Aero forces go up with the square of the speed. So at 150 km/h (air speed, remember, so that could be 130 km/h road speed and a 20 km/h headwind), the rear downforce created by the wing has increased to (150/100)^2, or 2.25 times. That is, it has increased from 8 to 18kg.
Given that I am
already gaining true downforce from the undertray, the wing adds 'too much of a good thing' - especially when its downwards force is behind the rear axle line (and so helping to lift the front).
Therefore, I typically run with the wing producing little or no downwards force. (But it is still needed to hold the fins vertical.)
(And at 200 km/h and set for maximum downforce? 32kg (70lb). And that would definitely be too much of a good thing!)