I used to have mine in the centre of the screen, as low as the mount could possibly go. I did notice that it was sitting on the back seat after an MOT test, though…
It was the best position I found for visibility at the time, however, now I use a Nite-ize Steely stuck to the front centre of the dash, just above the centre console; much easier to read and adjust, if necessary, there’s much less cable floating around, and it’s much neater and tidier than a big lump of plastic stuck to the glass.
Just have to be a bit careful where I put the phone, seeing as how it’s got a high-power magnet stuck to the back…