Don't worry if your glaze comes out translucent. As it dries, the colour will become more white. It will also be mostly absorbed by the cake.
I used about 3 large lemons to get 1/2 cup of zest. I did not press the zest down, so it it is a very light 1/2 cup. The number of lemons you'll need varies on the size that you buy.
Please keep in mind that nutritional information is a rough estimate and can vary greatly based on products used.