I was wondering if someone could help me.
I wanted to build (or let a company build it) a square rimless tank from acrylic, but i'm not sure how thick the acrylic needs to be.

I found the Acrylic Thickness Calculator, but I am not sure if the thickness that the site says I have to use applies to square tanks. Because the site doesn't ask for the width of my tank, and that's important to with a square rimless tank, right (?).

The dimensions of the tank that I wanted to build are 50cm x 50cm x 50cm (20" x 20" x 20" rounded up).

Also, the site says : ''KEEP IN MIND: The calculator is designed for water volume. Not air displacement. So while a tank might be 24" tall, it only really holds 21-22" of actual water. This is yet another thing to consider when inputting your dimensions into the calculator.
Example: i know when i build a 20" talltank, it will really only hold 18" of water..... so i enter 18", not 20". ''

Is it bad bad that I don't pay attention to the air in the tank, and I pretend that it is 100% filled with water. The acrylic willthen become thicker but that will only make the aquarium stronger, right? Orare there disadvantages to it, except the price of the acrylic.

OH! I also wanted to build some bridges (I want to connect 3 aquariums of the same dimensions). Do I have to pay attention to other things, when it comes to the thickness of the tank when I want bidges to connect them?

I said in the beginning that I also considered too let a company build it. I live in The Netherlands, and I haven't found a company that knows how thick the acrylic needs to be for square tanks, doesn't matter what size. So going there for advice is not really helpfull. Acrylic is not used(much) in The Netherlands for tanks, that's why a dutch fishkeepers forum isn't helpfull either hahaha.

I hope you guys could help me.