Yes, except for a log-log plot I would use a power law trendline. I use the high and low price during the quarter versus the ending BV, but the starting BV is just as good.
One issue with this approach is that the trendline value depends on the starting date of the regression, so one has to do some kind analysis to determine the best starting date. Using a starting date of Dec 1999 I get a trendline value as of June 30 of $266 (1.27x BV); using a starting date of Dec 1981 I get a trendline value s of June 30 of $289 (1.38x BV); using a starting date of Oct 1964 I get a trendline value as of June 30 of $328 (1.56x BV). I think that about $310 (1.48x BV) may be the best estimate based on Buffett’s repurchases.
Buffett has said that he would not buy back stock at a “mere” 5% discount to his estimate of IV, and in March, 2022 Buffett repurchased stock at an average price of $323 (1.40x March 31 BV), before pausing in April and May when the stock price rose, and then resuming purchases in June. If $323 is 95% of Buffett’s IV estimate, then his estimate of IV as of March was $340, or 1.48x BV. The IV (trendline) that I got as of March 31 by plotting price versus BV starting Dec 1981 was $342, or 1.48x BV. I think that this is a pretty good guess.