I think you are cutting it pretty close esp. this cycle which has been so crazy. Healthy sperm in the best conditions can live up to 5 days. I'm not trying to worry you, but since you are learning your cycles etc. I don't think charting etc. is the safest protection right now.
I really think that by the time you are getting a close to +opk, it's too close. I would highly recommend getting the book Taking Charge of Your Fertility by Toni Weschler. It's an excellent book and it does discuss natural birth control via charting etc.
And yeah, I'm kicking myself for the 3 years of bcps and double protection for the first couple of years because we wanted to wait 3 years to TTC.

If I had only known.... 4 years later.

If I had read the book, I probably could have saved a lot of money then.