Yeah you can't just do this with a consumer webcam out of the box because they're constantly trying to auto-adjust the white point. I mean not if you want anything "accurate". (Some webcams allow you to lock the white point in software though.)

But indeed, it is very non-obvious what you'd select for the constant exposure and white point (or combined as a gray point). You might also want to apply a strong curve to the brightness so nighttime skies are quite visible.