I'd add that the measurement is just a preliminary step: it gives you an idea of what size to try on for starters.

When you go to try it on, most places will dump a couple of 10 pound sandbags into the bottom of the pack to "see if it fits. How's that feel?" My answer is usually, "Not like any pack I've ever carried on a trail - and I don't mean that as a compliment."

My own experience has been that I get the best fit if I haul the gear I intend to carry (including the typical amount of food and water I'll start a trip with - 2 days' food and a quart of water for me, usually) to the store, choose the pack I'm interested in, and the size I think I'll need, and then load all my own gear into the pack. This does two things.

First, it determines if the pack will hold all my gear, with room to spare for those atypical trips where I need to carry extra water or a few more days' food. It also determines whether things will go where I want them to go (will the maps fit into the lid? Will the rain gear fit in the outside pockets? Will the water bottle fit in the "water bottle pocket'? Will pack straps or shoulder straps interfere with access to a pocket while I'm wearing the pack?

More importantly, with the weight distributed realistically, I can put the pack on and see how it's really going to carry. I can tweak the loading to get the side-to-side, front-to-back, and top-to-bottom distribution right. Then I can start adjusting the torso length (if adjustable)and other fit features. If I can't get it right, I can change pack sizes (from medium to large, or vice versa) or even pick out a different pack, until I find one that fits - then I can carry it around the store for half an hour or so to see how it feels after carrying it a while. Then I buy it.

Two etiquette items here: 1) Especially with a small store, check first to find out when business is slow, and go then - it's rude to scatter stuff all over their sales floor and monopolize a clerk's time when they've got a lot of other customers. 2) After you've spent an hour or two on this process, don't be a jerk and say, "Gee I really like this pack, but I saw it on sale on the internet for $15 less, so I'll buy it there." Pay the extra $15 and patronize the folks who went out of their way to help you find your perfect pack.