How can the radar range for ATC be estimated based on stated maximum radar range?
A radar system designed to track artillery shells is likely to be entirely useless as an air traffic control system. 

To improve their utility radar systems reject signals outside their design parameters. This means that stationary elements (buildings, landscape features, etc.) are ignored, as are objects with velocities well outside the target range. There may be other optimisations.

The muzzle velocity of an artillery piece is typically well in excess of the speed of sound (Reference) A radar system designed to handle artillery shells may reject many subsonic traces, and thus ignore most aircraft that you might want to have some control over.

Of course, you could reprogram or repurpose the electronics, but then you wouldn't really have a naval radar anymore.