Hi Dauntless
Part one: you can't change the sensitivity of RadarBox to pick up only flighte within, say, a 5 mile range, but you can set alerts to do that - click the Alerts tab, click the Home button in the middle, and set the required range, tick the Range box above it, and choose what alert you want.
Part two: the Polar Diagram shows how far you're picking up aircraft. Click the button with the yellow outline star at the top of the screen, and zoom out from your home location, and you'll see the jagged line surrounding you, which is the maximum range that you're receiving in all directions. It's an historical record - the longer you have your box running, the more chance there is of aircraft filling the gaps.
You have probably also spotted that when you move the mouse pointer to to any aircraft on the map, the distance and bearing are shown at the bottom of the screen.
Hope that helps
Rod
