Air Conditioning, Heating & Refrigeration News, Jan. 6, 2014, by David Richardson.
http://www.achrnews.com/articles/125242-using-duct-surface-for-leakage-testing

Author questions "the metrics that have been in place for residential duct leakage testing for years. Are they the most accurate metrics to use for determining duct leakage in a residential duct system? Would this be the best metric to use for quantifying duct leakage in residential duct systems?"