This is interesting and starting to look impressive. If you have 30 seeds potatoes, the row is 30' long and you are going to get three rows in the width, then there will be 3' between spuds in the row. You only need 18" or 2' at the most, so you could buy more spuds or do two staggered rows.
Are you intending to use this bed as a way of clearing the ground for other veg next year? If so, the muck/straw/ seaweed layer can be fairly thick, say 9" if you have enough. Seaweed is notorious for disappearing as it rots, so don't count the depth of that in your total. I wish I had access to seaweed - I have to use the dried stuff.