Dwc questions! Design suggestions needed

I'm building a grow cab out of two sterilite containers and I had a few questions. Ok, the containers are going to be stacked ontop of eachother. The bottom one being right side up, and the top being up side down.

Now my question is what would you say is a good minimum amount of water for the reservoir to hold? .

What I'm trying to do is use the bottom container as a reservoir, while keeping it as low to the bottom as possible. Less water = less height. OR, I can have a seperate container as the reservoir, and use the 2 big ones for the plants.

Any suggestions? The sooner I get this figured out, the sooner I can start growing again.

Well for one what's the size of your sterlite container ?

18gal it's 15 inches deep

I'm also thinking since I have a smaller container, I could turn this into an ebb & flow. It would probably save space too.

Based on my experience using a 18 gallon rubbermade tote when I was flowering with it with 10 plants I would use 15 gallons. And when I flowered 6 plants with the 18 gallon rubbermade tote I can get away with 10 gallons but I was changing my nutes out every week religiously. I hoe that helps you out.

i personally aim for 4-5 gal per plant but 3 gal works really well