Progesterone

What is considered low progesterone? I’m about to be tested on CD21 for progesterone levels while on Femara. I’ve been ovulating on it (clear temperature change on bbt and positive OPKs), but doctor wants to check if I have low progesterone. Can you have low progesterone and still ovulate? I guess that’s what confuses me because I had always heard if your progesterone is tested and it’s low then that means you never ovulated that cycle. Anything helps 🤷🏻‍♀️