The problem with trying to time your toking with UA's is everyone in the thread would be different. So many variables with getting clean. As I stated earlier the only way is to test yourself regularly to see how your body reacts. But it will never be 100%. Residues can build up and bite you in the ass when you least expect it and then end up with a failed UA.
People should think long and hard before they risk it. I feel sorry for them because a little toking could certainly release some of the pressures of being on probation. There could be a discussion on maybe the whizzinator or other tools that are designed for this sort of thing.