TC. The basic idea here is.......Displacement determines overall weight. If you have a bubble big enough to sit in, it takes X amount of weight to make it sink. You can either use a thin walled pressure hull and strap on a bunch of lead, worrying that you may accidentally reach crush depth and die, or you can spend the same amount of money on a thicker hull, use less lead, and be assured the crush depth is deep enough that you'd never approach it.
The work involved is the same. The overall weight is the same. The plumbing systems are the same. the electrical systems are the same. Thinner ( cheaper) windows still puts the crush depth too close.
There's really not much sense in building for a shallow depth when the added cost is such a small percentage of the total.
It's more logical to build it strong, even if you never go past 60 feet.
A 5/16" hull without stiffeners won't go anywhere near 60 feet without crushing. More like 18 feet and it buckles. Run some numbers through the calculators you've been given.
Remember that a pressure tank is designed to withstand internal pressure like propane. To break the tank is to reach "burst" strength of the steel.
A pressure hull for a sub must withstand external pressure. The steel doesn't "rip" or burst. It merely has to bend in such a way as to make the tank collapse.
Imagine how easy it is to bend a piece of steel just 5/16 inch thick. A pair of Vise grips will bend it. A small ballpeen hammer will bend it. Smack it with a 2X4 and bend it.
Steel is actually cheaper by the pound than lead bricks. It's cheaper to use thicker steel than to buy a bunch of lead to make the same size bubble sink.
-----------You've been given good advice, and pointed in the right direction. Get some books, learn a few basics, and I can assure you your new questions will be quite different than the ones you've posted so far.-----------
It's fun to dream and we encourage people to think outside the box, but we're pretty big on safety around here. What we say, recommend, or suggest reflects on us as a group, and the personal submarine sport as a whole. I may sound a little overly conservative, but the last thing anyone needs is BAD advice when we're talking about a potentially deadly activity like building your own submarine.
Good luck in your quest. It's a long term project and you may have taken your first steps. It's a long road.
Frank D.