I met up with Dave on the river and here is the result.
Here is a scanned copy of the the above article as published in the January 2013 Gear Guide issue of Fly fisherman magazine. If not, here is the link: http://www.flyfisherman.com/feature/fly-casting-overcoming-the-tailing-loop/
In part 1 of this series, the weight of a spey line that properly loads a rod was shown to be a weight dictated by power and tempo. The analogy used for this proper weight was a favorite book between the book ends. Moving on from this, we'll now be looking at the max and min length of the line akin to the two book ends holding up the weight proscribed above.
However, let's tackle one book end at a time, with the maximum spey line length for this Part 2 piece. The max length starts with- but does not necessarily fully comprise of- a length the caster can carry in his D loop just before the D loop catches water and sticks behind him. But before we fully get into this discussion, and because this out-of-water max length is highly dependent on how deep or shallow the caster is wading, let's assume the caster is standing in 30" of water so as to not confound this max length any further.
Picking a proper loading spey line for your spey rod is not much more clearer than when I started spey in 2005. Part of the problem is today's commercial offerings provide one size fits all lengths without giving a clear guide how to fine tune them. This only leaves the typical angler wanting to get his proverbial spey line wet just as discombobulated as when I started. My aim in writing this article is to give that person perhaps some understanding what makes a line properly load one rod but not another; you know, giving the tools how to catch than actually catching the fish for the reader.