New pool owner, trying to get my chemicals balanced

[PoolDoc]

I need to update the Best Guess page, to use a more % based dosing.

However you cannot follow the dose curve all the way down to 0.1 FC at 0 CYA, because of the fragility of very low FC residuals: any sunlight, and almost any chlorine demand will eradicate the residual, leaving the pool unsanitized.

Also, chlorine + UV is much more effective at destroying organics and complex chloramines than chlorine alone.

The kinetics of the Cl-Cya molecules is such that, when you have a HIGH CYA level, there's is a functionally limitless reserve of 'true' FC available. This is a factor I need explain more clearly on the Best Guess page, too.

[BigDave]

... does the CYA effectively bind up a lot of the chlorine such that it's there, but not always available until the CYA-chlorine bond separates? ...

Pretty much exactly. In a pool with chlorine and CYA most (almost all) of the chlorine will be bound to the CYA and some will be available for oxidising stuff in the pool (also available or destruction by the sun's UV). There's an equillibrium between the available chlorine and the CYA bound chlorine - as the available chlorine is consumed more is released from the CYA. The CYA can be thought of as chlorine storage, releasing available chlorine when and where it's being used. The equilibrium point depends on the ratio of chlorine to CYA, if that ratio's small then here will be very little chlorine available for cleaning the water.

@PoolDoc: I think you meant unsanitized in the second sentece of your post.