Low Progesterone Investigation

Hi all! I went to my doctors this morning & she's scheduling blood tests to find out why I have a short luteal phase & really short light periods since my miscarriage in March. Unfortunately I'm cd6 so she said too late for the first half of the cycle blood test. She said to book in for 7-10 days before I expect next AF. My luteal phase is only 9-11 days so should I aim for 7 days before just in case it's a really short month? What is everyone's experience when you've done these tests? I want to make sure I go on the best day!! X