Torso: That C7 vertebra is the bump (usually prominent) you feel at the back of the neck where it meets the shoulders. The bottom of the torso measurement should not be at your waist but the level of your iliac crest (the top of your pelvic girdle, aka hip bones. ( You can feel your pelvic girdle by pushing your thumbs downward from your waist at your sides until you hit bony resistance. If this is the way you measured, you should be OK.

In most instances, if you're a "between" size, it's recommended that you go for the larger size. Or, as suggested, you could order both sizes and send one (or both!) back.

The hip belt should not be on your waist but rest on the top of your pelvic girdle. The idea is to transfer most of the pack weight to your pelvic girdle (hips), not your spine.

Some short people have a long torso, while some tall people have a short torso. Torso length is not necessarily proportional to height. Some of us, for instance, have a long pelvic girdle (mine is long, while my torso and legs are both short).

Be sure to have all your gear, including the equivalent in weight/bulk of a week's food and a day's water, on hand before the packs arrive. Leave the tags on the packs, load one up and take a several hour's hike around the house (you gotta keep that pack clean and dry in case you need to return it). It will be the most boring hike you ever did (try music), but after those 2.5 to 3 hours, your body will tell you if this is the pack for you. You may or may not need to do this with both sizes of pack--it may be obvious when you put on the empty pack which is best. However, it's extremely important to be sure the pack works for you and your gear when fully loaded.

There are lots of articles on the home page of this site on gear selection, including this one on backpack selection.

There are also a number of articles there on reducing pack weight and an excellent model gear list ("27-lb, 7-day pack list") should you wonder why we experienced lightweight (not ultralight and certainly not "stupid light") backpackers are commenting on the size/weight of your pack.