OPK progression

This was my first month using OPKs. I used the Babi brand which was approx $6 for 20 tests (included shipping) from amazon or eBay. If was stressful. I expected a positive around CD 12/13 and was confused when it got a tad darker on those days. Luckily I kept testing and got a positive one this morning CD 16. How helpful! I thought I was ovulation earlier!