0".
No need for a flue damper IF you have a well sealed "airtight" wood stove, an EPA wood stove, or flue with a good draft.
Dampers "damp" the draft, make it difficult to brush the flue, can coat with creosote, and the spring will deteriorate and break in time.
If you have an older stove (or furnace ) that is either pre-EPA (~ 1989), or leaks air, or your flue has too much draft, a damper will help.
There are FIRM opinionated opinions here about this ( see one from a confirmed damper person from Iowa ! ). :hmm3grin2orange:
HAPPY NEW YEAR !!!!