Considering the most common keys for guitar music, and assuming capo use, the 3-5-7-9 deal has always seemed weird to me. Because when capoing at fifth fret, which of course is common since it puts you in C in standard tuning (or G in DADGAD), all the markers are in the wrong places: 2-4.

That's why I have developed the antisocial habit of fourth-fret capo at least for some solo pieces I do. The scale length seems comfortable there as well--you get a good reach, but it still sounds like a guitar not a mandolin.

W-O