Try on a number of different makes and models of packs. I depend heavily on load lifters because the tops of my shoulders are extremely pressure-sensitive--I need to keep the shoulder straps pulled up and off the tops of my shoulders. However, the back length of my pack is the 15" torso length. (I can't recommend the pack I use because it was discontinued in 2006.) And the load lifter angle is 45*. A lot of packs have load lifters at a different angle or have no load lifters at all.

You need to get your correct torso length (or the next size up if you're "in between" for that model of pack), but you also need to find a pack with effective load lifters.

Be sure to take your gear with you when trying on packs, and ask for an appointment with their most experienced fitter.