Could have been two things he was talking about.
You can roughly judge distances on the clipboard quite easy: Grid lines are 25 miles apart, the radius of a radar range circle is ~12 miles
But as you said you dind't catch all it could also have been about shooting with the cruiser's 8" guns at land targets using "Land Mode" and clipboard.
If you enter that mode while in ship's guns, you can simply click on the point on map where you want your shells to fall.
For more on that topic, see
http://trainers.hitechcreations.com/bigguns/bigguns.htm