Well, since as far as I know we have no way to change how firmware senses the battery level, not much we can do about it. For what it is worth, in my case it started chirping right after it hit 77%. But even if it takes a while between it hits 77% and the chirping - in the end, it is just an alert - it is up to you to actually change the battery. If in your case it will take a while, you can take your time changing it, but at least you’ll be ready. Personally I’d rather change batteries a little early, than have it chirp at 4 am.
I would be curious as to what the curve signature of rechargeables is - I bet you can probably detect them dying and alert ahead of time. While conventional wisdom about them not being safe may be right, it does not take into account being alerted when, or even before, they die - and even if you have to charge them twice as often, with smart notification reminders, that may be worth it.
In any case, my next purchase will be hardwired Nest Protect’s instead of these 