Once, on the bottom layer, you can have 20-footer(s). Once you go to 40-footers you can never go back in the stack. They're stacked 10-high above the deck. So

Once, on the bottom layer, you can have 20-footer(s). Once you go to 40-footers you can never go back in the stack. They're stacked 10-high above the deck. So you can have <10% 20-footers. Or put them all on the bottom, and only switch to 40-ft once.