"Secular bull markets can start only from low valuation levels. US equity market multiples are currently a whole lot more like 2000 than they are like 1982. It would seem to me that if multiples start high, then a medium-to-long term trend of lowish price increases is rather more likely than a trend of highish price increases."

Agreed. And how, if at all, will a medium-to-long term trend of lowish price increases for the S&P 500 affect Berkshire's BVPS growth and stock price? The data since 1965 look to me like BRKA's price growth and BVPS growth (BVPS growth is not shown in the table, but it is very similar to the stock price growth.) will qualitatively track the S&P 500's growth, although at a higher growth rate. (In my forecast I used the same growth rate, 8%, for both the S&P and BRKA, before changes in the S&P's P/E and Berkshire's P/B.)