The Screen Balance is set automatically after picking the Screen Color.
Saturation is measured by comparing the intensity of the primary component against a weighted average of the two other components. This is where the Screen Balance control comes in. A balance of 1 means that the saturation is measured against the smallest of the other two components in the screen color.
A balance of 0 means that the saturation is measured against the larger of the other two components. A balance of 0.5 measures the saturation from the average of the other two components.
The appropriate balance point for each image sequence you key is different depending on the colors in that image. Generally speaking, blue screens tend to work best with a balance of around 0.95 and green screens with a balance of around 0.5. These values are selected automatically the first time you pick the screen color. If the key is not working too well with these settings, try setting the balance to about 0.05, 0.5 and 0.95 and see what works best.