There are very few countries that have reversions. There are really very few precedents, but the question is exactly your question: Where is the sweet spot? If you go too low, then you will get push-back saying that it's not enough. It really comes to down to a compromise, so there is no perfect....
It's like the term of a copyright or a patent. There's no perfect answer. You pick a number that seems to be in the right sweet spot, and 25 is I think a good number. It could be 20. It could be 30. It's somewhere in that range, I would argue.