Clicking on a vitamin or mineral and seeing a breakdown of its sources would be kind of neat, but fitday doesn't have that capability. You'd also be looking at a percentage of all (100%) of the magnesium you have so far, and then getting that breakdown. So it would change every time you added a new magnesium-containing food to the log.
Kind of like watching the percentage of saturated fat change as you add new food to your food log.
If you find a food and add it, adding 2 cups instead of 1 cup, the calories should be double when you add it to the food log. The nutritional breakdown, in percentages for the food, won't change. The nutritional breakdown for your total food for the day would change, in a report you can get from 'reports.' If I understand your question correctly....?