After watching that vid, I'm sad to see that it's no longer being developed and that the devs didn't leave any open source behind. :( That would have been really cool to try out.

Good rule of thumb to capitalize letters with circle is just to make the circle as large as it can go within the keyboard area. It doesn't have to be centered around the key, just needs to start on the key. For center I usually just circle down from the key as far as I can.