A proposal by global regulators to change how the largest banks report key risk metrics at the end of the year could deal a blow to their U.S. short-term funding businesses, the latest salvo in a battle over tighter capital rules. The largest global banks are required to hold more capital than others, with a surcharge calculated every year from Dec. 31 values of a set of risk measures. In a bid to reduce the capital hit, which hurts profitability, some banks adjust businesses at year-end to lower those numbers, a tactic called window dressing.