I suppose people need some examples.
Example 1: My last project tank.
The tank is a simple plywood box made from 3/4 A/C plywood. The max water depth is 22 inches. The inside dimensions with max water depth are 46.5 x 22.5 x 22. The calculator tells me that the tank is 99.64 gallons (not bad considering i was just guessing at 100). It also tells me that the water required to fill the tank will approximately weigh 832.02lbs. The biggest thing it tells me is that with a safety factor of 3 i should use 3/8's glass. I also checked what 5/16's glass would be rated at and found a safety factor of 2.
Example 2: Greenterras tank
Dimensions as provided by Greenterra earlier in this thread 92x45x31. The calculator tells us that it will be about 555 gallons. I chose a 3.0 safety factor which showed that the optimum glass thickness would be 11/16's. I then checked what the more common 3/4" glass would rate and found 3.9.
Example 3: Cvermeulens' 600
His stated dimension are 96x48x30. I am going to subtract 1.5 from both the length and width assuming the inside dimension are cut down by 3/4's on each side (the general construction practice) I am also going to assume he's not filling it to 30 inches but probably closer to 27. So, for the calculator I am using 94.5x46.5x27. It tells us that at that level the tank is around 525 gallons. He could be filling it higher, it could be built different.. i dunno. This is purely conjecture. However it seems to be very accurate as the calculator shows that .54 inch glass gives a safety factor of 3. His build is using half inch glass salvaged from a 135.
Hopefully this will shed some light on the calculator and its uses. It is a very good tool to assist you with your tank design. I am happy to answer questions and take suggestions.