- They don't take commonplace optics into account -- officers frequently had a set of binoculars. Modern (well, WWI) optical range finders were not yet in use, so you don't have a lot of help in ranging for artillery fire, but you can get a grip on the presence and nature of a body of men at a far longer range.
- What really interferes with visibility is not distance, but the clutter between the observer and the potential observable.
Now, lets limit that. Lets divide the world into flat and slope. That's more than a bit artificial, since the world is just not that flat. But our wargames tables are and that is what we have to work with. We'll also divide the world into levels, to lend some meaning to our hills. We won't worry right now about what sort of height difference we mean.
The point here is that, if you are in a hill or a slope and what you are trying to see is on a lower level or a slope facing you, you will able to see some over proportion of obstructions. Likewise if you are in a town with a convenient church steeple.
On the other hand, while you can see a regiment in column of companies easily enough, men in an extended line who are taking advantage of the terrain will be far harder to see. Unless they are shooting at you; there is no smokeless powder in this war.
So how does this turn into rules? I'm not sure yet, this is just where my thinking is right now.