I haven't tried sewing packs, so my fitting is limited to dress bodices, where I do try to fit the slope of my shoulders. BTW, the slope is not the same on either side! Most of us are not symmetrical.

If you are making the pack especially for you, you certainly would want to do the same. I hope you can get some help with the fitting!
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May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey