Hope this might help you. The following was provided to me by Sabine Contreras, Canine Nutritionist. I provided a link to her site, she has been a great nutrition advice source for me.
Quote]You are correct that the amount of calcium you need to supplement depends on how much is already on the recipe, but it also depends on two other things:
1. how much in total is needed to meet the daily RDA of the dog in question (which is a non-linear progression based on body weight) and
2. how much is needed to balance the phosphorus content in the food to an acceptable level (around 1.3 to 1 is ideal for most normal healthy dogs, but up to 2:1 is generally also still acceptable[Quote
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