Back in the day I had a tax prep guy (enrolled agent, actually) that would use the midpoint of the price range for the year. He kept copies of the WSJ for the last trading day of the year.
Thanks, we decided to look back at the historical prices and pick dates that are the average of the year prices. Our accountant said it's up to us to provide the basis price for the stocks. This stock has not fluctuated much over the years. Pretty stable and has a small LTCG. ie, not making much. I doubt there will be any red flags.