Flydoc1:
George Harvey's objective with his leader formula was primarily drag reduction so he could use heavier tippets to protect the fish. This drag reduction was achieved by virtue of gentle "S" curves formed in the leader when delivered.
He experimented with all hard nylon and all soft nylon when it became available and determined that a combination of the two was the answer. His leader also had a lighter butt section that was .015 (sorry Tim
😉).
He did tie a 10-inch "attachment" section of leader material to his fly line that was .017, but
he DIDN'T count that as part of his leader.
You were close in your assumptions, but most of the adjustments George Harvey made to tweak the performance of his leaders depending on fly size, was in the LAST hard nylon section that was .009 (2X). This section was either shortened or replaced with soft nylon and the soft nylon sections to the tippet adjusted accordingly.
With that in mind, approximation, speculation and assumptions are great, but sometime you just have to go to the source if you want the real answer...
From his 1985 book, "Techniques of Trout Fishing and Fly Tying:"
"I start with a small piece of hard nylon (about ten inches) tied to the line with a nail knot. This makes a smoother juncture between the line and leader. I don't count this as part of the leader. My basic leaders are tied with twenty-inch sections of hard nylon in the following sizes: .015, .013, .011 and .009. A supply of these basic leaders are carried at all times and are then adjusted with soft nylon as dictated by the size of the fly I am using.
The following lengths of soft nylon are approximate because the air resistance and size of the fly being used determine the actual length. When I taper to .007 (4X), I cut off the .009 hard nylon and add soft nylon in the following lengths: 12 to 15" of .009 (2X), 18 inches of .008 (3X) and 22 inches to 28 inches of .007 (4X). This gives me a leader of approximately 9-1/2 feet.
When I go to .006 (5X), I cut off about half of the .009 (2X) hard nylon and add 12 inches to 15 inches of .008 (3X), 18 inches of .007 (4X), and 22 inches to 30 inches of .006 (5X). This leader will be approximately 10-1/2 feet. If you want to shorten the leader, take several inches off the .015, .013 and .011.
I have been using this leader formula for over thirty years and rarely use tippets smaller than .006 (5X) and most of the time I can take trout on leaders tapered to .008 (3X) or .007 (4X)."
FWIW - NO tippet material brands are mentioned in the book. When George developed his leader formula in the 1950's, "Mason's" was a popular brand of hard nylon. Garcia Platyl was one manufacturer of soft nylon at that time. I'm not so sure about other available soft nylon leader material.
Not knowing George, but knowing that generation of anglers very well, I can almost guarantee he used a micrometer so he KNEW the diameter of the material was what he wanted, not what was printed on the spool.
I hope this helps!!
