When I follow someone, I notice it's difficult to keep a person fully on my iphone 7 screen. Do youhave some tips to keep a person fully in my frame (screen) ? I don't want to correct my holding position on the gimbal all the time or use the joystick.

Besides adjusting gimbal settings, I think this is something that's just gonna take some practice to get better.  Keep playing around with it until you find something that works for you.