Water Profile - Calcium 50-150ppm?

Just curious where the recommended range for Calcium range of 50 to 150ppm comes from?  Referencing Palmer's book...

From Palmer's Water (page 147) : "Calcium is the friend of all brewers who brew with alkaline water. The reaction with malt phosphates is one of the primary mechanisms for the mash pH drop. It is remarkably flavorless. It protects, stabilizes, and promotes enzyme activity in the mash. It aids in protein coagulation, trub formation, oxalate precipitation, yeast metabolism, and yeast flocculation. The calcium levels in the water need to be high enough to carry sufficient levels through the boil and fermentation. A range of 50-200 ppm in the water for the mash is recommended."

Found this over in BA:

Below is what Brad Smith discusses concerning Calcium in brewing water:

?Calcium (Ca)

Calcium is the primary ion determining the ?permanent hardness? of the water. Calcium plays multiple roles in the brewing process including lowering the Ph during mashing, aiding in precipitation of proteins during the boil, enhancing beer stability and also acting as an important yeast nutrient. Calcium levels in the 100 mg/l range are highly desirable, and additives should be considered if your water profile has calcium levels below 50 mg/l. The range 50mg/l to 150 mg/l is preferred for brewing.?