How to calculate weeks?

Hi mommys Did some research online and they say to calculate weeks based on last menstrual period date. But for sure I know I ovulated late this month (probably after cycle day 24)…. Last menstrual period is 10 December Conception date first week of Jan Only tested positive after missing period for 6 days My period cycle is usually about 30-35 days. Not exact.. How do I know how many weeks I’m in? Thanks in advance