I don't believe I have seen this link posted yet and it surprises me (unless I just missed it). It is a calculator to determine acrylic thickness for a custom sized tank. Near the bottom is a link to an excel spreadsheet that all you have to do is plug in the l x w x h and it will give you the answer.
http://cyro.custhelp.com/cgi-bin/cyr...YWdlPTE*&p_li=