Top of the zone/base of the sternum is about a 0.5-1 ball lower than shown IMO
Could be. Without X-ray vision it's kind of hard to pick out a random bone in the human body beneath flesh, muscle, clothing...but that's what the college rule says is the top of the zone.
By the way...this illustration isn't something that I cobbled together with Photoshop. It came straight from the NCAA rule book.