Phil uses RIO's 15-foot Powerflex leader for the foundation of his long leader setups when using the 'naked' or long line nymphing technique with floating or Midge Tip lines.
The butt section of this leader provides support when casting long leader rigs, 15-feet or greater. To make a leader longer than 15-feet simply add an equal or one size smaller diameter tippet. For example, to make a 20-feet leader add 5-feet of tippet to a 15-foot Powerflex leader.
When using long leader tactics you must factor in the differing sink rate of the leader along its length due to the thicker diameter butt section. To ensure your fly or flies sink deep your leader must be 25% longer than your targetted depth. For example, to reach 16-feet your leader must be 20-feet long (16 x 1.25).