From what I gathered from your e-mail, the job isn't really of such importance that if you fail the test it'll be the end of your very existence. In that case, I say fuck it -- do what you do. If you can get another job that's just as good, just as easily, then at least you'll have this first screen as a trial run for the next.
However, if this is not the case and you smoke like you say you do -- daily. And if 3 weeks is the cut off. I can say with about 75% percent certainty that you will fail. Most of the people that dilute actually recieve a NEGATIVE DILUTE and that is permissible in some exams. These may be the people suggesting dilution because they can get away with a diluted sample.
I couldn't. And they just sent it out to a lab to further analyze it. So now you have the advice, run with it.
And if you decide to use your own piss -- you'd better run away from the blunt at least until test time. Good luck! Let us know the results!