Submarine project hull strength

I am thinking of building a propane tank submarine in a year or so and I'm trying to get some different perspectives on hull crush calculations. I've been researching the project for a few years and realise that hull crush depth can be difficult to calculate accurately. Basicly what I want to do is make sure my submarine hull does not crush in during unmanned testing which will probably be about twice the max diving depth. I also checked out one or two other related threads on this site and it seems like the topic isn't super common. That's to be expected, not everyone is building submarines.

Anyway, I have a friend that was an engineer and did work with ships, buildings, ect. He is better at figuring the math out than I am and also knows some of the terminology a little better, so I asked him to do a test calculation on a plain propane tank. No stiffeners, ribs, hull openings. Just a 500 gallon propane tank with semi rounded end caps. I guess they are called 2:1 heads. (Not perfect half spheres)

Specs he used:

Material - Mild steel

Thickness - 3/16" or about .190" (Heads and cylinder)

Diameter - 37"

Length of cylinder (to tangent of heads) - 119"

Using the formula for external pressure vessels on this site, he came up with a calculated crush depth for that tank of 177' fresh water or 80psi. I am just wondering if we are close to the mark or not. I know it takes some time to run through, but I'm just trying to make sure we're on the right track. Thanks.