“I’m not sure I understand what you mean by " I ask for the simplest to utilize because we likely won’t be using NFP to space our babies, but instead as a tool to see when the conception occurs and thus when to expect our next baby to be born.”"
I mean that, unless a serious reason presents itself when my fertility naturally returns as the babies nurse less and less, we will not be using NFP to “delay” or “achieve” pregnancy. Just want to use it more for awareness.
We choose to go with the least amount of intervention possible during pregnancy, and will not use ultrasound imaging unless it is medically necessary. From what I understand, the margin of error for fetal size is quite large, and many women have been induced early based on faulty assumptions after an ultrasound was performed.
I predicted my due date based on when I felt I ovulated (symptoms) and had kept a record of my periods for 7 months before marriage. My date based on when I assumed I ovulated and the date based on the calendar were a week apart. The babies were born 2 days before my predicted date and 9 before the calendar date, so they were either 39 or 40 weeks!
Before you think I’m too crazy, we did have a homebirth midwife, and she went by the calendar. She also did basic prenatal care, and my healthy, full-term twins were nice sized and alert after birth, and have never even had a cold!
“On the other hand, if you’re only interested in dating a future pregnancy, the easiest “method” would probably be to just take a pregnancy test every four weeks and make an appt for an early ultrasound once it comes back positive. Of course, I’m so pro-NFP that I feel like a bit of a heretic for writing that…”

That actually sounds like a good idea (uh oh, now you’ll really feel like a heretic!). Would you believe I never actually took a pregnancy test?! Since I conceived on my first post-marriage cycle and early pregnancy symptoms were obvious, I felt no need to buy a test! Of course, things are different now, and now that I have 2 nursing babies, something besides intuition might be necessary!
“So I think you’re looking at mucus-only methods like Billings or Creighton, and if it were me, I’d go with whichever will be easier to learn because of proximity to an instructor (maybe even one who makes house calls!).”
I will have to investigate those methods. The only one I knew about was the symptom-thermal, and I do not want to set an alarm to wake up to take my temperature! Maybe if I had a reason to postpone, I would, but for my purposes, it seems like a lot of trouble (and the babies might wake up early!). So thank you so much for mentioning those mucous-only methods! I see another poster mentioned Billings as well!