Fix, Principles of Brewing Science, 1999, pg 5
Calcium ions also tend to afford thermal protection for mash enzymes (Comrie, 1967). IN addition, they continue to interact with malt phosphate during wort boiling, and the ongoing reaction between calcium and phosphate is the primary reason that the pH decreases in the kettle boil. Calcium ions also tend to inhibit color formation during the boil and facilitate protein coagulation. Finally, calcium ions also influence beer fermentation. For example, they favorably affect yeast flocculation and beer clarification during maturation (Harrison et al., 1963; Saltukoglu and Slaughter, 1983; Taylor, 1990)
He goes on to say (on page 6):
A widely accepted rule in brewing is to have calcium concentrations of at least 50mg/L, and values in the range of 100-150 mg/L are very common.
Anecdotally, I've done mashes with water around 20 mg/L Ca++ and achieved 85% efficiency and I've fermented meads with 0 mg/L Ca++ that have achieved final gravities putting them into the dry range from an initial gravity north of 1.100.
However, when it comes to brewing, if you're going to take 4-8 hours of your day to make beer and George Fix and other sources of contemporary beery wisdom have accepted as an axiom that 50 mg/L Ca++ is desirable why not just aim for that as a minimum? Seriously, I'm asking - is there any reason compelling reason, other than anecdotal evidence, that suggests the best beer is made with Calcium concentrations below 50 mg/L or above 150 mg/L?