Two spectrums of thought on it, but it's definitely not the conservative way to do it - a lot of those investments are hampered by somewhat onerous investment restrictions brought on by insurance regulators. And, if you look at almost any insurer in the world, you could make the argument that for every $1 I invest in their common stock, I get $3 of investments. Why subtract investments from the stock price for Buffett, and not for every other insurer?
The counter-arguments to that are that Buffett and Prem are much better investors than normal, even in fixed income, so they should get more credit than normal. Also, some of those investments are outside the insurance subs. Long story short, if you are going to subtract the full amount for Buffett, it's worth considering why BRK-A is preferred to other P&C insurers valued on that basis.