honestly if i were you i would stop right now with dillution because yes, you can overdose on water...
not only that but it's a huge waste of time, if i were you i would enjoy a great nights rest and wake up around 6 am, drink 7 glasses of water and 3 energy drinks, then take alot of b vitamins and creatine monohydrant, i would also pee at least twice before going in, when you do get there and are about to let it fly, pee the beggining of the streem in the toilet and the middle in the cup then finish the rest in the toilet, reason being is there is most thc in the first shots of the streem
hope all goes well
-captain