When to use OPK

When do you use your opk tests? My fertility dr has me starting every month on CD11, and she says to pee once when I wake up in the morning, then test with my second urine 2 hours later. I always thought you were supposed to use FMU, but maybe I was wrong? My doctor says FMU is too concentrated for OPKs. I use regular Clearblue (NOT the purple advanced digital). Anyway I’m just curious if anyone else has heard this or was told something different?