I think if your gear fits in the pack and the pack fits you (and carries the weight without causing soreness), it's a good pack to have.

You need the measurement between the iliac crest and that vertebrae that sticks out when you bend your head forward to dial in the size of the backpack.

If your size fits in the middle it's a guess whether to go with the next size up or the size down. I go the direction that gives me a little room for adjustment - if the straps and load lifters have to be cinched allllllllll the way up for the pack to fit, time to go to the smaller size.