Take Overlord suggestion seriously, since a mistake in defining a pressure vessel geometry and thickness can have serious "consequences".
I have to underline that also the formula in your file, that determines area "A", seems quite odd: you get as a result a negative area, this should be carefully checked because it isn't definitely what one would expect from an area
I think that this thread is giving you the chance to dig deeper into the proper equations to be used and their meaning, since by mixing equations from two different references there is some risk of using them in an unproper way.
ASME VIII div 1 or EN13445-3 section 15 will guide you through a more complex but safe design, where both corners and shell midspan will be checked against membrane and bending overstresses.
Concepts as corrosion allowance and joint efficiency (to correlate between adopted thickness and Non Destructive Test after fabrication) are also included there.
And you get requirements for allowable stresses as well, which is an important topic in the pressure vessel industry.
Adopting a proven design and construction code will be required in case your vessel needs to comply to some pressure vessel safety regulation (based on the country where the equipment will be installed, or any other contractual agreement); you might be required to CE stamp (EU), or U Stamp (US and many other countries too..), or have compliance with EAC regulations (Russia, KZ, Belarus), etc...
If this is the case, again, you better stick to a coherent pressure vessel construction code (as the main ones mentioned above by Overlord).
In case you want to dig into any other source\book for rectangular vessel design, you may want to check:
- Dennis Moss "pressure vessel design manual" (check every equation again... remarkable book in terms of tons informations, methods, and logic behind, but there is some mistake here and there)
- Bednar (cant' remember the exact title), with more details about the derivation of a formula, while Moss is mainly a design handbook
... and many others
In the end, I found some time ago an old book which is titled "Tubular Structures", including a very clear and clever way to determine wall thickness and stiffener dimensions for rectangular powder silos, bins and bunkers. It was a very interesting reading (besides typos, some equations needed some adjustment..) and, even if powder is not going to behave as hydrostatic pressure at all, the book gave me a useful insight on how to model properly the structure with a "by formula" approach.. finally it was a good way to understand better how rectangle section silos are checked inside Eurocode (EN 1991 for structure, i.e silos).
Bye