Ok I will try to just get directly to the point on this one. I am a board certified toxicologist, so this is an area that I know a great deal about. You provided a lot of helpful information, however it is still not an exact science on how long the half life elimination of the drug from your system will be.
Of couse the standard 30 day period is thrown out there, because that is the longest amount of time that it takes for almost anyone to clear THC out of the system after an exposure to the drug.
One other thing that can throw a "wrench" into guessing whether you will pass or fail, is the type of instrumentation that the testing lab will be using to test your urine. If you go to a place that uses an "in house" test kit, then these kits use a procedure called TLC (thin layer chromatography) and they are less sensitive, so you have an easier chance at passing this type test. If you go to a place that collects your urine and then sends it out to a clinical testing lab, then it could be analyzed on a variety of different scientific instrumentation, such as a TDX instrument, a GC or a GC-MS (<---this one is the most sensitive and will pick up very trace amounts and is therefore very difficult to pass-however it is expensive to use, so most routine drug screens are not performed via this method).
If this is for school, or for a job, and you have not yet had to go in and take it, then I would run down to your local drug store and buy a hom test kit. Go ahead and do the kit on yourself and see whether or not you pop a positive on that (this will be that TCLP type). If you pop a positive on that, then you have some worries and are going to need to do a lot of exercising, drink a lot of water and stay away from fatty foods until the test, and you may squeeze by. If you do not pop a positive, then I would still err on the side of caution and steer clear of fatty foods and keep that metabolisim up and you should be fine. Do not even bother with any of that cleansing stuff if your urine is being sent out to a lab, because it will not fool a GC, TDX or GC-MS.