"Mixture Calculator"

Hi, I'm sorry to bother you all but I'm in need of some assistance. I need a calculator or spreadsheet that can (trough approximation) determine the percentage of certain components (within an error margin) needed to have a specific final solution. An example of a possible use would be creating a medicine or a feed for an animal.

You have your list of ingredients but each ingredient has a list of nutritional details and these may have another sub list.

You want a final solution with A amount of energy, B - fiber, C - protein;

From the proteins you want a specific amount of a few amino acids.

I would like anyone that knows any programs that are capable of determining this to list them here. Especially any open source programs.

Thank you for your time.