the bottom piece is calculated with the sides sitting on top of the bottom sheet, and the bottom sheet being supported by the stand at four corners (like a tubular steel frame supports a tank).
I think tanks SHOULD be fully supported on the bottom with a strong piece of plywood anyways, though.
It's really just a "fail safe" calculation for your bottom piece so it doesn't burst out.
Again, I think the main thing the calculation doesn't account for is holes being drilled. Be careful you don't have too many holes, and if you have large ones, maybe upgrade to the next size up in sheet glass?
Note: I didn't design the calculation, I only addapted it to software to make experimenting with it easier. It was demonstrated by Warren Stillwell, and here's a link to his DIY article:
http://www.fnzas.org.nz/articles/tec...lassthickness/