The best time to replace the battery is when it's causing you trouble, in the form of either performance issues or poor battery life. The 80% figure is meaningless, that's just what apple says your battery should be at after around 500 charge cycles, but that varies with usage.

I've seen 4 year old iphones @ 73-75% battery health still getting very decent SOT, and I've seen 1.5 year old iphones @ 90-92% battery health getting absolutely shit battery life.

Is the phone not lasting as long as it used to the point it's interfering with your day to day activities? Change the battery.

Is the phone more laggy or stuttery than it used to be, perhaps getting ocasional freezes that bother your? Change the battery.

Ignore the battery health indicator.