I currently use 3 cups of DRY seed per tray. The black trays are 14.5" x 18" that's 261 square inches. I'm using seed straight from the combine, so some of that volume gets skimmed off the top when I'm washing -- don't need the floaty bits of straw and stuff in the fodder. Of course I soak the seeds before I put them in the black trays, so they expand and weigh more ... I also pre-sprout them for a few days. Not all the seed sprouts at the same rate. Because the grid on the bottom of my black plastic trays is too big, I only transfer the well-sprouted seed into the tray, though sometimes I then top-dress that with the less-sprouted seed, or I just feed the less-sprouted seed to the chickens. It grows thicker if I top-dress the well-sprouted seed with the less-sprouted seed, but it is also slightly messier as some of the seeds fall out the bottom of the tray.
Everything works better in the clear plastic food-prep trays which are about the same total square inches but different dimensions. This is mostly because the holes are smaller so the seed doesn't fall out the bottom before it sprouts.
When I put the DRY seed in the trays (this only works with the clear plastic trays), the seed densely covers the bottom of the tray. After soaking, the seeds are bigger so the seed layer looks deeper.