A slight darkening in the corners of the foreground layers will accumulate as each layer is composited. Never compensate for low light levels by boosting the gain on the camera. Boosting the gain inevitably produces an increase in noise in the image, and the noise will get amplified in the Ultimatte. A polarizing filter will be essential in any foreground shot which requires a forward falling shadow in order to match a backlit background scene. The only way to get a forward falling shadow is to have light striking the floor at an oblique angle to the camera.
Glare is inevitable, and a polarizing filter is the only way to eliminate it.
The camera should be balanced for the color temperature used to light the foreground subject. If a polarizing filter is used for a scene, the camera should be white balanced with the polarizing filter on. It is important to place the chip chart in a location where it is isolated from the backing so that it is not contaminated by bounce light from the backing. The camera lens should also not be pointed towards the backing so that there is no flare in the lens from the backing. If the white balance of the camera is contaminated by bounce light or flare from the backing, color of the backing will be misinterpreted by the camera when the foreground is shot.