I'm not exactly sure but I'll tell you what happened for me. I stimmed for 10 days. My e2 on day 10 was around 1600 and they counted about 20-23 follies with 15 estimated to be mature. On day 11 (after trigger - no stims that day) it was up to 3,000. Retrieval was on day 12 and they got 25 with 15 mature.
I think e2 is one piece of the puzzle but it isn't everything. Just because you don't have exactly 200 per follicle doesn't mean you won't get plenty of follies. Also, that 200 is per mature follicle so imo you'd have to find out your e2 level on the day of retrieval for it to be accurate since it keeps going up until then.