Martin suggested 40 ppm of Ca as a good lower value for the kettle (in the thread I referenced previously). I have previously been using 50, and this will help on occasion (like for bopils).
EDIT: As he points out later, I am wrong about the above. He actually stated that he doesn't think there is a huge need to make sure that there is at least 50 ppm in the kettle.
Thanks for that data point from Kolbach, Kai. I'll assume that's a pretty good ratio to use at the concentrations of malt and Ca we typically encounter in our mashes.
And yup JJ, you could intend your Ca kettle additions for yeast health, beer clarity, beer stone (more of a pro issue), or flavor (via the Cl or SO4 anions). Or all of the above.
It'll be interesting to see if the water book publishes any new proven data on any of this stuff.