Well, the safety factor is a number that all good tank builders use in order to keep tanks from breaking under certain conditions.
The variability of the strength of glass due to limitations of the manufacturing process means a suitable safety factor must be used when calculating glass thickness. The factor commonly used is 3.8. While not a perfect guarantee, it will remove all risk bar that of damaged or very poor quality glass. The main damage that will cause failures is scratches and chips. Also a point load on the glass surface will cause it to fail. For this polystyrene or other soft under tank fillers are used to stop the point loading of sand and rocks.
Calculating Glass Thickness for Aquariums
The website above gives a great example on how to calculate the ratios used in constructing tanks.
I hope this is useful, even though the information might be somewhat confusing and detailed.
At the bottom of the site are three calculators available for your use, one even for your Excel program for your PC.