My calculations were to keep this 6 years or more before considering a battery change. Looks like at this rate I will be at 60% in 4 years. That will seriously suck for utility, i.e., not practical or usable for me.
I'm not sure how you can make such a calculation. There are several factors here which you may need to consider:
1) Where was your car when the original owner had it? Was it in a warmer or a cooler climate?
2) The battery SOH drops quickly at first, then it drops more slowly for a while.
3) At about the point where the third bar drops, the battery SOH begins to drop more quickly again.
4) Some of the "loss" seen in the SOH number which is reported is really due to the car moving capacity below the low-battery warning level. I find that in the summertime I can drive farther than the SOH number implies. Unfortunately, in the wintertime, it seems to be pretty accurate. That is because one cell-pair of the 96 in our LEAF's battery seems to have a particularly-high resistance in wintertime (but not in the summer). Oh, if not for that one cell pair!
Our MY2011 LEAF is six year old with over 51,000 miles and still has three bars. Our car was at about that level of SOH at about the same number of miles. I live in the northern tip of VA, so it is cooler here, but your MY2015 likely has a battery which is a bit more robust. As such, and assuming your car was use in Tennessee by the original owner, you might
expect similar results to what I am showing in my signature.
This summer will be very telling for our LEAF. (Capacity of the LEAF battery is lower in the wintertime, but it does not degrade during that time, if that makes any sense.) It's at 47.4 Ah right now and the degradation has shown some acceleration over the last 10,000 miles. I'm wondering if it will lose the fourth bar this summer or wait until next.
Good luck with your LEAF. I'm sure you will like it better once the temperature increases a bit. We all do!