What if one never sold more, as a percentage of their portfolio, than the operating earnings yield of the past 4 quarters?

Sure, but it's a bit volatile.

Rather than the conventional definition of operating earnings, I would use the net earnings of the operating divisions. Handy after-tax numbers by division at the beginning of the "Management Discussion and Analysis" section of each quarterly and annual report.

The question is which subset to use. If you use them all, it's very close to just using the change in book per share--and volatile. If you want a relatively steady figure, ignore the underwriting result, the investment gains/losses, and the "other" category. Underwriting is too volatile to be useful as is, and "other" is unpredictable and in any case small on average. You could add dividend and investment income from the income statement, minus an estimate of taxes on that. You'd still be excluding any capital gains or losses, both realized and unrealized, but it would be much steadier. But by skipping the capital gains it would be conservative, since that does add up over time.

Jim