The answer to your question is %5 of current market value. All the other issues mentioned are emotional baggage. That doesn't mean you can't consider them. Maybe since you aren't too personally invested in the property and your brother spent a lot of time effort and money, you are emotionally satisfied with accepting %4 or 3%, or perhaps you feel cheated out of 25 years of fair use due to family dynamic, and would be more comfortable with 6%. That's up to you and your brother. But your question was which year is the %5 based on, and the answer is the fair market value of the property at the time you decide to sell your stake.

The answer to your question is 5 % of the current market value. All the other issues mentioned are emotional baggage. That doesn't mean you can't consider them. Maybe since you aren't too personally invested in the property and your brother spent a lot of time effort and money, you are emotionally satisfied with accepting 4 % or 3 %, or perhaps you feel cheated out of 25 years of fair use due to family dynamic, and would be more comfortable with 6 %. That's up to you and your brother. But your question was which year is the 5 % based on, and the answer is the fair market value of the property at the time you decide to sell your stake.