If you get much over about 2x what you feed, then the algae starts to spread out more across the screen and it seems that the growth doesn't get as thick, and sometimes doesn't grow green very well, but instead grows more light brown or yellow & gooey. I believe this is because the algae is in essence getting starved because it's spread too thin.
But you have to take all factors into account - mainly lighting and screen size in this case, but also flow to an extent.
So if you have an oversized screen but you only light part of it, and the amount of light you use matches up with what you "should" or "would" use if your screen was properly sized, then I would not consider your screen to be oversized, but rather, underlit, or something like that. Call it "technically underlit" but since the lighting matches the load, it's still proper. hard to put into words
jerseypete in your case, adding extra screen to the bottom is not adding active growth area so I wouldn't even consider that part of the calculation.
