OPK and FMU

Raes

So I took my first opk test yesterday around 4:45pm and it was pretty dark. I took a 2nd one today with FMU and it was a lot lighter. I just read that I shouldn't test with an opk with fmu- only pregnancy tests. So is it worth it to test around 4:45pm again today to see if it is as dark as yesterday?? TIA