When in my cycle to test for progesterone ?

I have noticed that for my last five cycles (avg. 29 days for me) my luteal phase is very short (only 7-8 days, and yes I’m predicting and confirming ovulation correctly with OPKs and bbt). I’ve been ovulating on Day 21.

It says if you have a short luteal phase it may be caused by low progesterone, and in order to check your levels, you need a blood test. To get an accurate reading on that test, it says you’re supposed to test 7 days after ovulation, which for a person with a normal period, is Day 21 of your cycle.

But since I’ve been ovulating on Day 21, does that mean I wait till day 28? Or would that be too close to when I get my period? I just want to know which day will be most accurate for giving me my true progesterone level.