If a chicken sandwich was all chicken then why would there be any percentages to worry about, proprietary or otherwise?
Food that you buy in the grocery store must have, by law, all ingredients listed in order of weight. Different common spices and herbs are just listed as spices/herbs without naming them.
If it has ingredients that can cause allergic reactions, soy, milk products, nuts etc. it must list them.
AFAIK, food that you buy at a restaurant chain does not......
If you buy a whole turkey at the grocery store, it’s likely to be 85-90% turkey, because they inject it with a brine to make it juicier. Chicken is often brined, too.