Oh, and I begin getting the usefulness of the flat cover.
If one uses the dome for determining the ratio, in the case implied by Matt with the fill light "frontal" and the main light "lateral", one must turn off one light to measure the other light.
If instead of using the dome one uses the flat cover, the different power of the two lamps will be read precisely, without need to turn off the other light. With hot lamps that might even avoid stress to the lamps (turning on and off in rapid succession).
EDIT In fact this was the conclusion that was already implied in post #13 by 2F/2F even if I did not catch it immediately.