Yesterday when I had my random I asked the tech what was there cutt off point for water consumption if I need to set there and hydrate to take the piss.He said 48 ounces is what they use.I sat there and drank 32 ounces and my urine was still pretty yellow.
SO just follow the normal guidelines for dillution:
Drink some extra water or Gatorade in the morning of the test.
Pee a couple times before the test.
And take a midstream sample of the urine.