Well lemme see here... glass is made several different ways, of course there is tempered but there is also standard glass that is made with different things such as iron etc that affect the clarity. We generally don't use tempered because in the event of a catastrophe tempered glass shatters and poof there goes your fish flying across the carpet as the water rushes out of your tank. For the sake of simplicity the manufacturers have fudged a bit with their safety numbers which is a good thing, we'd hate to advocate the bare minimum in security. With all of that said, while some clearer glass may be a bit more brittle the standard SF should more than compensate especially if you use framing and cross bracing.
You are welcome. Glad people find it useful. I tried to make it as malleable as I could to let people play with the numbers and get a feel for what their preference may be.
