mollie, as far as I understand chemical pregnancy is a very early miscarriage, possibly where you get a BFP before your period is due and then your period comes later than you expected it because the embryo has failed to implant, whereas a miscarriage is where the embryo implants but fails to develop so miscarries some time after your period was due.
The general advice is to test at 10dpo so you can either come off the pred if you're taking it from ovulation or start/continue taking it if you get a BFP. I was using the internet cheapie tests which I always find to be pretty reliable, even though the lines can be very feint on them before 14dpo. I think this time I got my extremely feint, but definitely there, line at about 11dpo and they work out at about 10p a pop which is definitely favorable to forking out for FRERs! This is where I get mine from www.homehealth-uk.com/medical/pregnancy-tests.htm - just the strips in a little foil pouch. They come in 10mIU/ml too which is the highest sensitivity. They also do OPKs which are nice and cheap if you're going to try without superov and you can buy them in a combo pack with the pregnancy tests, choosing your own combination.
I think you should be fine on the superov if you test early from now on. I actually used to cut it pretty fine with ordering my meds as I didn't want to spend the money on them if they weren't needed so I'd test early and only order them if I got a -ve the day before AF was due! Also, if you're really concerned speak to Louise. I used to sometimes start the meds a day or two later than others I'd seen on here and she said that wasn't a big problem so if you're worried about taking letrozole when you may already be pregnant that might be an option, just to make sure.
I'm sorry this one didn't work out for you. It's been a right old week for a few of you on this thread. I hope you all have very self indulgent weekends and enjoy some lovely wine.