I just bought my silicone from Glass Cages. You gotta call them to order it, but it will get to you in a few days. Takes all the guesswork out. You can get black or clear and they will tell you how many tubes you need and glass thickness. They were a little on the fence with my build of using 1/2" glass for a 24" tall tank.
My current tank was built by Miracles, and they reinforce the bottom to the sides with an inner rim of glass. Few inches wide but spans the length and also width of the tan. This way, if your worried the base doesn't seal perfectly, that inner "ring" of glass is easier to accomplish that. I think the bottom edges have the most pressure. (could be wrong about that thought. Seems to make sense) Anyway, it's a cheap way for some peace of mind.... There are also vertical inner glass strips on the back two corners. (top to bottom)
There's a video on youtube that shows a tank build using an entire sheet of glass a little smaller than the bottom. It's siliconed in place on top of the bottom piece, and the sides are put into the "half channel". This way, it gets a double seal and stronger bottom...looks like kind of a pain though so I'd just stick with the 5" strips if you're gonna do it.
I am a glass supplier...too bad you're not closer. I could help you out. Glass is not cheap if buying retail. To go from 1/2" to 3/4" is over double the cost. maybe even tripple.