That calculator doesn take a few things into consideration when building with plywood. One is the abundant bracing type reinforcement that plywood offers, the other is the fact that the glass is usually not bearing the pressure at the bottom seams, that is taken by the plywood. So in actuality you can use thinner glass than any calculator will show. This is because they are planning on using all glass, and plywood is tough to account for. I know of a few 30" tall plywood tanks that use 1/2" glass. You can do this because plywood is infinitely stronger than glass, and the bracing around each pane, and the fact that the glass doesn't bear pressure at the seams -esp the bottom where the pressure is highest.
