It’s more about being right on the cash flow, less so on picking the perfect discount rate. Try using 10%, it’s your opportunity cost %. You have BRK available ( or the S&P 500, you can argue…) to buy which will give you 10% - it’s your hurdle. Whether you use 8.1, 8.9, 10.1, it doesn’t really matter, the value will scream at you. That’s my un-academic view. Others will chime in on risk premium, WACC, long bond rates….it’s all over kill.