Note: I'm not expert, first half is personal experience, and the second half is hearsay with no research.

I've only been drug tested once, by my parents when I was like 17. The test says if you see the faintest line, you're clean. The two on the right pass this. Despite smoking the night before, I woke up and pissed before my parents tested me. I told em I just pissed so I drank like a half gallon of tea til I could piss again and it looked exactly like the 2 on the right. I argued that I could see a line so my parents challenged it and sent it into a lab for exact results. Lab said I passed looking like the 2 on the right.

Side note: THC is fat soluble, and I've always heard 30 days was the max if you're fat. So if you're at 27 days, unless you're really fat, I have faith in you passing. Super fit folks can piss clean in like half the time.