The correct way is to see how deep into a Pringle can it can go. For an accurate measure, you'll need to prevent angled entry. You can help it slide in straight by using a couple guiding sponges on either side of a latex glove attached to the rim pushed up inside it. Put a bit of Vaseline in the glove to prevent tearing the glove with the rough bark. For maximum safety, put the jar of Vaseline in a warm water bath for a few minutes beforehand to make the viscosity better for smooth entry and exit.