Interesting link to the resource. I have seen other references to calcium helping to stabilize the alpha amalyse (Malting & Brewing Science)(The Biotechnology of Malting and Brewing). Unfortunately, even the reference at the link above does not cite a minimum calcium concentration. The other references don't either.
Calcium is precipitated in varying degrees through a variety of reactions. From my research, the amount of precipitation is proportional to the calcium concentration up to about 110 ppm. Beyond that point, the amount of precipitation is again proportional, but on a lesser slope. This has no bearing on the minimum level, but it has a significant impact on the concept of Residual Alkalinity. That will have to wait for another day.
Regarding the minimum calcium concentration, there does not seem to be a good reference with respect to alpha amalyse performance, yeast health and flocculation performance, or oxalate precipitation performance. We know that malt contains some calcium and it can be somewhat liberated in the mash (especially decoction). I don't know what the minimum calcium concentration should be, but I think the evidence suggests that its in the 40 to 50 ppm range.