I took a home test yesterday with a 50 ng/ml, using my first pee of the day, and got a VERY faint test line (supposedly indicating a negative result). So, I'm freaked out because I'm thinking that means my concentration was under 50 ng/ml, but just barely.
Youre thinking incorrectly. There is no meaning attributed to the intensity or width of a line. Even synthetic urine controls (which contain NO drug metabolites at all) can produce a faint line on an assay. A faint line is still a line, which is a negative.

I have to take a test next week, and the place they want me to use is Legacy Metrolab in Portland. I've read some horror stories about this, mostly I think because they use the 20 ng/ml cut off instead of 50
The party who is paying for the U/A is the one who decides what the assay cutoff threshold should be - not the lab. The lab just provides the "test menu" for the client to pick and choose from.