What I described was for training purposes only. It's not practical as a general line of sight tool.
Once you've learned to read the ground, developed your coup d'oeil, you'll start trusting your judgment over any line of sight tool. In games that have them, I almost never use them. They just aren't as accurate as you'd think.
The general rule is, unless you're sure your opponent can't see you, he can. If you're going to try to use the line of sight tool to micromanage where you put your troops, and put them right on the edge of line of sight, you're not going to like the results, because just because a line of sight tool says there's no line of sight doesn't mean you won't be seen.
You can google coup d'oeil, but don't expect the results to be useful. It's one of those things that can be learned, but probably can't be taught. Given some of the stuff I've read, it can't even be explained properly. I couldn't understand it myself for many years until I got a game with a line of sight tool, but you can do the same thing during the setup phase to learn it there.
I'm not saying we shouldn't have a line of sight tool, just that you're better off learning to read the terrain. A real LOS tool would help, of course.
Hook