HCG levels after early miscarriage

Hi,
I'm after some advice because I can't find the answers I'm looking for. I had an early miscarriage/chemical pregnancy on the day I would've been 5 weeks pregnant. I got my BETA 23/11 & hcg was 312. I had blood tests on 30/11 as I had started spotting & my hcg had dropped to 180 & my progesterone was 7, on 2/12 my levels had gone to 190 & 5. I saw my OB 8/12 who confirmed everything & did an ultrasound which looked good & he couldn't see any retained tissue, I'd bleed for 5 days with 2 days of spotting before & after that so he didn't see the need for a d&c thankfully.
I've had positive OPK's all this week, which tells me I still have HCG in my system but I'm wondering if anyone can provide any insight on how quickly it will drop to 0 so we can start TTC again? This was our first natural pregnancy after 2 successful ivf pregnancies & 1 failed cycle, so it was a miracle & very devastating but we're hoping we can get another miracle that sticks! We've been seeing a natural fertility specialist as we won't be doing any more Ivf so we are really keen to try again.

TIA for any info.

I read that if a natural miscarriage takes place over an extended period of time, the rapid HcG drop could happen at any point between the point where baby stopped growing and when you actually began bleeding. However, it may take another couple of weeks to drop it to zero.
Ideally you should wait for at least one period before you start ttc. This is because most of the time accurate information on LMP is necessary for estimating proper due date, gestation age etc.