The best way to gauge jacket length...

[Rob]

... would be, I assume, the back measurement?

And taking this a step further, I would imagine the back measurement to be from the base of the collar, to the bottom of the jacket (ie: where it's sitting on, or over, your butt)?

(Can anyone tell I'm finalising measurements for my Wested? )

[Bufflehead Jones]

You are correct. Measure from the seam at the base of the collar, straight down the center of the back to the very bottom seam of the jacket.