Fertility blood work- P4 Progesterone

Hi ladies, I've been told to schedule a blood draw to check on my P4 levels. Doctor told me to schedule it when I start PMS or a week before my period but I don't quite have a regular period every month, sometimes its 36 days cycles/other times up to 48 days. Don't always experience PMS either.

Anyone who has experience have a guidance on when exactly is best? If I use opk to track, how many days after ovulation should I go in.

These blood tests are expensive where I live, wouldn't want to waste it.