For years, many pension funds assumed they would earn an average 8 percent annually from their investments. Now, the question isn't whether return expectations should be lower, but by how much. The average target has fallen toward 7.5 percent, but that's still probably difficult to meet without taking excessive risk. Howard Marks, the co-founder and co-chairman of asset manager Oaktree Capital Group, threw out a number this week that was markedly lower and challenges models used throughout the pension industry. "If you walked into a pension fund today which had no investments, and you were given a pile of cash...