12 watts for 18 hours a day, 1-sided, handles 1/2 cube.
24 watts for 9 hours a day, 1-sided, also handles 1/2 cube.
Your 30 watts is a bit more than you need for 1/2 cube a day if you run it for 9 hours. But if you run it for 18 hours you can do 1 cube.
3 square inches of nori is about 1/3 cube. So you are wanting to feed a total of about 2 1/3 cubes. Thus, you do not have enough wattage. If your screen were 2-sided (with 15w on each side), it would handle twice the feeding with the same wattage.
If 30 watts is all you can use, and you have to keep it 1-sided, you'll need to run it for 18 hours to be able to feed 1 cube.
A good screen for your 30 watts would be 5 X 6, with a reflector so that all the light hits the screen.