Thanks - +/-20k tallies with my rough calculations based on measurements too, so I'll stick with that as a working number. Pool is now clear of all leaves, and the water looks crystal clear to me, but I'm still losing 2ppm overnight. Last night I was at 16ppm, and this morning I was at 14, so 12.5% loss. The night before was approximately the same (I don't have my numbers in front of me). The filter still needs backwashing every day or two, so there's clearly still something in there that's not visible to the naked eye.

Given that I'm still backwashing fairly regularly, and therefore replenishing water from time to time, would it make sense to leave the CYA until the filter runs for rather longer, instead of diluting it when I top the water up?

At 12.5% overnight loss in an unstabilized pool, can I start letting the level drift down yet?

Thanks as always...!