To OPK with FMU, or not. That is the question.

So people say not to test with FMU for ovulation tests. But what if you ovulate overnight? My sleeping schedule isn't a normal one due to work. And I've been seeing some people saying that they got a positive opk in the morning. I'm worried because I had a really dark opk one night, and didn't test in the morning because people said not to. Waited 4 hours after my FMU and it was barely a line there. I know lines fluctuate in visibility, but I'm worried that I ovulated and it was overnight. That's going to make tracking ovulation a bitch.