In theory an obvious method for assessment of a packaging design is market observations. Especially for products on the market you may have practical observations able to demonstrate that you are not able to reduce your packaging material because your sale will be reduced.
If you have such expensive observations and are able to document lost sale you can use this as demonstration of that you have reached a critical level.
If you do not have such observations you can reduce your packaging material on purpose until you can document lost sale. Such documentation can be accepted, but it will be an expensive method and other assessment methods are better used.