Goodness I'm getting so confused... First you hear that a DSB should be no less than 4" and no more than 6" now 12"?! Aarrgh ::ulls hair out::::
Don't sweat it too much. If you keep it at the 4-6" range you will be fine. My boss put a lot of them in, ( and he tragically died in '07 ) and most of those sand beds are still in operation.