First of all, I don't think Bob Marley could even come back with a 28 microgram level of pot in his piss. The level is measured in nanograms per milliliter of urine. If you had a level in the micrograms they would think you gave them bong water. As far as guessing if you are clean after 6 weeks, you may want to include you height and weight so we can see if you are overweight, your activity level (exercise does help you get rid of it faster), and your smoking history (how many days a weeks, how many time a day, how much, quality,etc). Drinking lots of water will not help you get rid of THC any faster. Drinking a lot of water the day of the test will help you dilute but it will not increase THC elimination from the body.

Best thing to do, get a home test and if you fail it go with the friends urine.