For years, personal finance advice came packaged in tidy percentages that were easy to remember and reassuringly simple to follow. Then housing, groceries, cars, insurance, child care, and other essentials became much more expensive, while many of those famous rules stayed frozen in place. Prices have continued rising even after the enormous increases households absorbed earlier in the decade. The result is uncomfortable but useful: some classic money rules don’t work these days, and our rules may need to change.