Using the usda's report of whole prey we see adult mice have a dry matter basis of 32.7%. So a mouse that is 10g has a dry matter basis of 3.27g(3.27g is 32.7% of 10g). Of the 3.27g 2.98% is calcium. That means out of 3.27g, 0.097g is calcium(0.097g is 2.98% of 3.27g). So in the end a 10g mouse contains 0.097g of calcium or is 0.97% calcium as fed.
I don't think I explained it that great but hopefully it makes sense.