Vintage value comes from the interaction of multiple factors
With vintage gear, value is not calculated by simply adding percentages for age, rarity and condition. The factors interact.
A rare but heavily altered example may appeal to a different market from a common model that is exceptionally original. Important provenance matters only when it is verifiable and relevant to buyers.
First build the item profile: identity, authenticity, originality, modifications, condition, completeness and demand. Then look for genuinely similar comparables.
For the full pricing methodology, use the Used gear pricing and valuation guide.