On this page you find a number of images which might help you in calibrating your cam for filming another iPhone. Don't use this to film an ordinary scene!
Use this screen to focus and expose/whitebalance properly for screencamming this device, the gray is a color-neutral 50 percent gray with some focussing/aiming aids.
Check for correct exposure and screen brightness, no blown out highlights or too dark shades?
Check again for correct exposure and screen brightness?