According to value investing, the general way to find the instrinsic value of a company, and therefore its stock, is to estimate its future cash earnings in perpetuity.
However, this doesn't take into account the company's book value, the net value of its assets minus its liabilities, namely debt. Warren Buffett, the world's most famous value investor, recently divested Berkshire Hathaway's shares of 4 major airlines, citing that they had taken on too much debt, selling them at a loss, since those airlines' stocks are selling at historic lows. This seems to indicate valuing future cash earnings for some time period, and then estimating proceeds from its liquidation (again, all discounted to the present).
What are some principles or rules of thumb related to this that other value investors use in valuation?