Rope angles on clay liners, never looked that close till now

I was doing a routine sweep last Tuesday over in Maplewood and decided to actually measure the rope angle on an old clay liner before I started. Turns out it was sitting at like 12 degrees off vertical, which I always figured was fine. My grandpa's rule was always keep it under 15 or you'll cook the liner over time, but I never checked one that close before. The homeowner had a camera inspection report from 2019 that showed hairline cracks near the top, and I bet that angle made it worse with heat rising unevenly. Has anyone else measured liner angles on older houses and found stuff that surprised you, or am I just overthinking a 60 year old chimney?