bro, i have this computation sheet for thickness of acrylics used in fish tanks, and yours falls under 12mm if braceless -- since you have purchased and done up the tank, probably it is safer to add bracing. You can download the file here....
http://www.mediafire.com/?9h6vi6cbvv6xjwv
I am also at the midst of doing my own tank - - have built 2 frag tanks so far, a sump undergoing leakage test (for office reef tank) and another sump for my home under construction. As it is my first time to deal with acrylics, tested first on small items and going big -- my last and upcoming project is a 3ft x 5ft tank.
I am also doing my own timber stands -- purchased fine solid timber pre-cut and assembling it. It is a fun learning process and I am lucky to have a patient wife as my living room turned to a factory.