I know this is a very old post, but I have the same question. I believe you're right, jetpack width should be measured by the top pic (widest point of the jetpack) not the bottom one.
Is 15" wide still what we'd consider "correct"? I know it depends on height, and I'm 5'8". I'm thinking this size might be fine.
EDIT: I printed out the templates and it's 15" in the top picture [USER=8597]RealMatix[/USER] provided.