What’s the harm in asking them to diagnose your battery and not even say anything about wanting a free swap? See what they say and document they had a chance to diagnose things before the warranty ends.
My feeling and expectation is Apple would not offer to replace things unless you have them perform diagnostics and they see something amiss or feel it’s about to fail. I also see +- 5% FCC estimate when I run things from day to day, so I wouldn’t worry about +- 1% either to be eligible for a swap on the pooled plan (you are pooling risk - so it’s not free, it’s paid for out of the pool). It’s also not free since you would waste a 80% good battery that might last for years if your assumption that the only thing wrong would be measured capacity below 80% FCC within 3 years of delivery.
The 80% FCC is the standard, so it’s understandable for you to look at things around that as a line, since Apple established that design line and goal, but in practice, 60% FCC still gives a lot of run time and sometimes it’s not even worth paying for batteries below 40% or less.
AppleCare plans are pro-rated and you’re paying for the term benefit (just like paying in for insurance of any kind) and no one gets to collect for “close to needing care” or a refund if you don’t use the benefit. Of course, you can cancel the coverage any time before it ends (and then the return is pro-rated). If you really think through the coverage, I think you’ll join me in “not wanting to get something for your payments” since the FCC is just one aspect of performance.
I would hope Apple wouldn’t replace the battery unless they feel it’s not performing, so I would just get it service diagnosed now and then about 2 weeks before the end of the coverage (or anytime you see 75% FCC). Since Apple can run diagnostics remotely , you can get a quick diagnosis and chance Apple would want to swap out parts (maybe something other than the battery needs service).
In the end, I gladly pay for battery service when it’s needed and if it takes 3 years to get to 80% FCC you might have another 1-3 years before the charge falls off and only have AppleCare and a $90 or so service for 6 years of productive use of a Mac spanning one original battery.