Does the gauge measure the hackle feather from the pin you bend it
round, in which case it includes the thickness of the hackle stem?

Yes, but that doesn't matter.

There you go! If the OP is actually concerned about whether the stem is
included in the measurement, I recommend drinking some anejo until it no
longer comes up in conversation.

I make this same recommendation in virtually *ANY* circumstance =)


As for using a gauge, I find it most useful for selecting and sizing
saddle hackles prior to attending a tying show/venue or going on a road
trip. I strip and size a few dozen hackles from a few saddles then bag
them in ziploc sandwich bags labeled by size... usually a range of 2
sizes (14-16) (16-18) so when I get to a show or am out camping/fishing,
I can be more productive. Then I take along full capes in dun, grizz,
brown to have a full range of sizes in the event I need them.

I also found the comment attributed to George Harvey interesting. A few
years back, I started tying smaller flies on larger hooks, which is sort
of similar to what was described. The difference is I tie from the bend
forward to about the 2/3 point of the shank and leave the front 1/4 to
1/3 bare behind the eye. This results in a smaller fly with greater
hooking ability, and it doesn't impact how it floats at all.

Larry