Normally, I think not chasing pH is good advice, but yours is too low. :(
I do like the idea of dosing Kalk. I saw a very interesting debate in the last day or two on R2R on dosing Kalk. The members involved never did agree but I was thinking that a combination of their two ideas would be great. Mix up fully saturated Kalk and use it as an Apex controlled additive using a small pump. Set the Apex to add just a little less per day than your normal ATO system would add. Let your kalk free ATO make up the additional water volume in case it varies. Dose your 2 part to make up for any shortfalls from the kalk.
This way you are dosing a constant amount of kalk per day so you can also dose a constant amount of 2 part. Your ATO would only make up for fluctuations in evaporation.
Did I explain that ok? :confused: