Where does the Thumb start from Metro Detroit?

Where does Metro Detroit end, and where does the Thumb begin? Both geographically and culturally

Would you consider Port Huron or Lapeer part of the Thumb? Does it start North of I-69?

What's your thoughts?

Edit: some ppl say Macomb County or the Southern part of St Clair County (Algonac, St. Clair, Richmond, Romeo, etc. is the thumb. To me those places are way too metro feeling and too close to Detroit)