You may want to consider visability and hand coordination. Even though the radio is adjusted less often, I mount it in middle (or left) because I fly with my right hand and I can adjust devices with knobs using my uncoordinated left hand. Mostly this is for changing volume because some guys' radios in my group are very loud and some are very soft. Detailed hand inputs like the navigate require my right hand, so the screen is on the right, off the panel. The vario is top center for best visual and also for left hand adjustments including volume.