Max,

Can't answer your question directly...it's a subjective answer. If 25ppm is too much, isn't 24ppm ALMOST too much? If you can be in 20ppm for one hour safely, how about 10ppm for two hours...shouldn't that be the same exposure? So, it would be impossible to pinpoint a number and say this number is safe but 1ppm higher and it's not.

I can comment somewhat on your pool conditions. You appear to be consuming quite a bit more Cl than most posters on this forum. azhobbs usage is more typical.....you may have some organics still in your pool that shocking would clear up. Secondly, you don't post test results so I cannot comment on your calcium levels nor any of your other levels.

Lastly, I think that managing a pool with high CYA levels is troublesome. It presents you with problems just like the one you are asking about...how much chlorine is too much? Painful as it may be, I think biting the bullet and doing a partial drain to rid yourself of the high CYA makes pool managment (and especially algae management) much simpler.