I'll give you the answer in heights using a 3.5 safety factor. In each case, normal (not execessive) bracing is assumed to be used:
26" -13mm
30" -15mm
36" -18mm
1/2 inch thickness (12.5mm) is often the thickest people indicate they can actually find due to the much smaller market for thicker glass.
If you use 13mm glass on a 30" deep tank, your safety factor drops a lot (to 2.5.) That might work if you have new glass from a good glass maker and also good bracing. If it's less on any of those you may have issues. Extra bracing might help improve that safety factor.
Also, keep in mind that it's the water height (waterline to the bottom of the water that counts.) If you intend to keep the top inch or 2 inches unfilled (some people do that), then that does not count as part of the height. E.g., a 28" tall tank with the top 2 inches unfilled, still needs just 13mm for a 3.5 safety factor, the same as a 26" tall tank.
