Motivation of copyright: Produce more works / Make the profession feasible for income
With this, I think (avg. time for a new work) * x would be a good choice. I do not like "death" + y because early works violate the above. Likewise, death + y or z is equally unfit because the longer one is chosen which again gives lifetime copyright for early works.
For avg. time for a new work, I took 5 years. As x I chose 3, so I took the poll option 15yrs. I do not know if this is enough time selling books to make enough profit but the reasoning is clear and I'm not fixed on a specific length, just the formula.
|