In 1990 Buffett used a 3% loss stress test, rather than a 10% for WFC. From the 1990 letter:
Of course, ownership of a bank - or about any other business - is far from riskless. California banks face the specific risk of a major earthquake, which might wreak enough havoc on borrowers to in turn destroy the banks lending to them. A second risk is systemic - the possibility of a business contraction or financial panic so severe that it would endanger almost every highly-leveraged institution, no matter how intelligently run. Finally, the market's major fear of the moment is that West Coast real estate values will tumble because of overbuilding and deliver huge losses to banks that have financed the expansion. Because it is a leading real estate lender, Wells Fargo is thought to be particularly vulnerable.
None of these eventualities can be ruled out. The probability of the first two occurring, however, is low and even a meaningful drop in real estate values is unlikely to cause major problems for well-managed institutions. Consider some mathematics: Wells Fargo currently earns well over $1 billion pre-tax annually after expensing more than $300 million for loan losses. If 10% of all $48 billion of the bank's loans - not just its real estate loans - were hit by problems in 1991, and these produced losses (including foregone interest) averaging 30% of principal, the company would roughly break even.
A year like that - which we consider only a low-level possibility, not a likelihood - would not distress us. In fact, at Berkshire we would love to acquire businesses or invest in capital projects that produced no return for a year, but that could then be expected to earn 20% on growing equity. Nevertheless, fears of a California real estate disaster similar to that experienced in New England caused the price of Wells Fargo stock to fall almost 50% within a few months during 1990. Even though we had bought some shares at the prices prevailing before the fall, we welcomed the decline because it allowed us to pick up many more shares at the new, panic prices.