Please refer to Ben's Best Guess CYA chart. The issue isn't whether your particular pool has never had a problem at 80 ppm (and what FC?), but whether ALL pools will be guaranteed to not get green algae if they maintain a certain FC at a given CYA level. That's what the chart is about. Your pool may not have nitrates or phosphates in them so you won't get algae because of that even if your FC is lower than the Min on the chart (for your CYA), but that doesn't mean that everyone's pool will be the same. In my early days (before I found this site and before I figured out some pool water chemistry), my pool got to over 100 ppm CYA with only 3 ppm of FC and didn't get algae, but I was using algaecide regularly and had the pool covered frequently.

Also, you can absolutely have rather high CYA (even close to 100 ppm) with relatively low FC (say, 3 ppm) and not get algae if you use a weekly dose of PolyQuat 60 algaecide, but that is more expensive. It does let you use Trichlor tablets in a floating or inline feeder instead of having to add chlorinating liquid or bleach every day. So there are options, but they have differing pros/cons.

There has not been one case, and I mean literally one case, of green algae in a pool following Ben's chart -- and that's with chlorine alone and no algaecide. There have been reports of the heartier mustard/yellow algae, but we figured out the higher level of chlorine needed to keep that away (essentially the Max column in Ben's Best Guess CYA chart or the 0.07 column in my chart (the 0.03 column is the minimum to prevent green algae and 0.05 is the typical column that is followed and is in between Ben's Min and Max columns).

One of these days I would love to create a tub or basin with equivalent pool water (i.e. with calcium chloride and sodium bicarbonate) and add nitrates and phosphates for a great algae bath and then see exactly how much chlorine it takes to just barely inhibit the algae (green and mustard/yellow) in their best growth conditions. That's an experiment that this industry should have done a long time ago, but never did. One guy did do an experiment, but didn't maintain FC levels and didn't ensure there was no ammonia in the algae extracted from its broth and he "proved" that CYA didn't matter (you can guess why -- the amount of monochloramine that's formed is independent of CYA and it kills algae). His study was funded by Olin Corp.

Richard