I agree with everyone, in particular I like yummyk's approach. I didn't have this issue with N-400, but I had it I think with H1-B and I think my lawyer solved it at the time by saying something like multiple short trips or something like that, as I couldn't remember how many land crossings I had done. It's up to you if you want to spend time with the airline, I don't think it is needed. Other suggestion if it makes you have more peace of mind is to check your credit card records, if you used your credit card during your trip it might help you pinpoint the dates you were out of the U.S. In particular if you used your card at the airport.