Minimum is 12 total watts per 12 sq in, so you could go 12w on each side of a 24 sq in screen, but 24 per side would be better. If all that light is "captured" by the screen, then you can go 1/2 time on the photoperiod (9 on). If not, you can probably still reduce it until you get green instead of yellow (which is too much light per nutrient delivered).

Acrylic shields will not block light significantly enough to impact growth. Acrylic is clearer than glass so you're good to go there.

Bio-load and stocking calcs have no meaning for sizing a scrubber. Only feeding as the input.

I would only oversize the screen by 50% max and then feed at that level. As you add fish, feed the same amount. Marine fish can go 3 days without food so feeding them less daily will do just fine, and when your quantity of fish increases to the point where you wish to feed more, then you can add to the scrubber. This will likely be months or possibly a year or so down the road as you stock your tank and your fish grow. Also you might be able to just extend the photoperiod a bit (if you're running 9 on) to compensate for additional feeding, but this may only work to a certain point.

As for your original problem, just drape Saran Wrap over the tube and extend it about 1" down on either side of the screen, and all your spray problems will disappear. You can also drape the entire screen in saran wrap and do away with the acrylic shield completely.