Letrozole and Ovulation

Hi everyone! Trying to understand what’s happening here. I’m on cycle 2 of letrozole; taking on cd3-7. My OB said I will ovulate sometime between cd11-14 however I don’t think that’s the case. I haven’t ‘peaked’ or had a true positive yet. Also I’m normally a 45-50 day cycle. Just started tracking using opks and bbt last month so no real history to share. Also taking provera to induce a withdrawal bleed. Last month I saw the same lh surge on cd8 but assumed it was not a true positive and possibly induced my the letrozole. Last month I got my first true positive on cd18. Should I be having timed intercourse based on my OB’s suggestion, or use my opks and cm as a better gauge? Any help is appreciated! PS I’m nearly 39 and have no children and was pretty much put on letrozole because of my highly irregular cycle and my age.