I've got longer fingers than just about everyone I've ever met, but when I pick up my Note 4, my pointer finger seems to be in that general area, but I agree it'll take some getting used to after having the scanner on the home button of the Note 4. If it works as quick and as well as they claim, I'll be all over it though.
I'll admit that the scanner on my Note seems to have improved after the last update or so, and by quite a noticeable bit, but I'm looking forward to not having to swipe to scan.
Love the idea of the low-powered Sensor Hub (is that what they called it) taking care of sensors and taking strain off of the main CPU too.