I dont really think there is a minimum Lynn. Coralline grows well in low light conditions, as long as the calcium, {strontium & magnesium also}, levels are decent. I have seen the best growth under vho or T-5 type lighting or even a bunch of 40w lights.
Higher "k" halide bulbs also seem to promote a better growth. When I ran 400w Radiums, it was thick on the back of the tank. Bulbs like 65K Iwasaki bulbs, seem to produce a lighter colour and more of the green type or not much at all, at least in my experience. But corals like it.
I would say if one was running actinics for the normal 11-12hr. period and main lighting for something less, then the growth should be fine.