Sustainable Food Calculations

Our project does not include any restaurants within the scope of the LEED project boundary, so we are looking to include all catering purchases made by each company, not individuals (our office tower has 3,500+ occupants). Everything has to go through security, so documenting (receipts) should not be a problem. My questions is, how do you identify whether a particular meal choice is fully sustainable or not when it's made up of multiple ingredients? Example: A pizza uses organic sauce, but the rest of the ingredients are conventional and the caterer may opt to not participate in tracking where each of their ingredients come from. Would you break the cost of that product apart or consider the entire meal meeting the sustainability criteria? When you compare this credit to other similar ones (most commonly found in NC (materials & resources tracking), they account for a percentage of the product to meet a particular standard. (I.e. 10% of PCR + 20% RRM in Carpet tile.)