I'm using Bleachcalc--one of the original chem calculators that was developed by MW Smith several years ago.

I think you'll find that even with the high CYA levels, if you don't let the chlorine drop down so low, then it really won't take any more bleach to maintain the higher levels than it would in a low CYA pool, because although you have to reach a higher level, you're also starting at a higher level. It doesn't take any more bleach to go from 0 Cl to 10 than it does to go from 10 Cl to 20. The problem you're seeing is that if you let it go all the way down to 0, then it take a BUNCH of cl to go from 0 to 20!!

I intentionally run my pool at a CYA of 80-90 for that same reason, because I live in a very hot, humid climate, and my pool is in full sun from sunup to sundown. I actually use the same (or maybe even a little less) amount of bleach as those with lower CYA pools, but only have to add it every 2 or 3 days because the stabilizer protects it so well from the sun. But it only works because I keep it consistently above the minimum laid out in the Best Guess Table. It also works for me because my pool stays open and chlorinated year round, so I don't have to deal with the CYA breakdown problem.

There is something else you need to be thinking about, as we get closer to the end of the season, which is the other consequence of too much CYA in the pool......depending on how you treat the water through the winter will have an effect, but basically that CYA will do one of the following:
1)Remain present in the water and you can resume the high Cl routine next year
2) Lower gradually over the winter IF you're not using trichlor and IF you keep it chlorinated enough to keep algae out and IF you either purposely drain some or do many backwashes or
3)If allowed to go green, the algae and accompanying bacteria may break the CYA down so that you actually start the year with a CYA of 0, but one of the byproducts of that breakdown is ammonia, which creates a HUGE chlorine demand upon startup of the pool that takes the same amount or even more of bleach to overcome.

So--it's not magically going to go away, you're still going to have to make a decision about either a partial drain or taking your chances with it breaking down over the winter. Personally, I would take the opportunity with every backwash to dump as much extra water as you can each time--believe it or not, it will slowly start to come down.

Janet