RC Chick's problem is very real. Whether you have one good eye or two, depth perception is a tough nut to crack. The problem is that our original manufacturer didn't think we'd really need to tell how far away something was past maybe 100 yards.

Consider how we regulate depth perception. Our brains do a complex math formula requiring the triangulation of our eyes, focus, and size. But the problem is, once you get past about 20 yards, focus is about the same. Don't believe it? Look at a camera lens. It will have distance marks for 2ft, 4ft, 8ft, 12ft, 24ft, and then the symbol for infinity. Your eyes work the same way. But, if you have two good eyes, you have the advantage over a camera of triangulation. your right eye will be angled slightly to the left, and your left eye to the right. Your brain does the math and calculates distance. But, unfortunately, this is very limited also, because once an object is about 20 or 30 yards away, your eyes are both pointing virtually straight ahead, so the difference in the angles of your eyes at, say 100 and 200 yards is virtually undetectable. So, we rely on our third option, size. Looking down the street, we can see that the streetlights are getting farther away because we know that they are all the same size, yet as we look at them, they seem to get smaller and smaller. But with a model plane, we have nothing to gauge their size with. If, for example, there were a plane identical to yours that you KNEW was sitting directly above the tree line, your brain can easily detect whether your plane is in front of it, or behind it. But without such reference, you can only guess.

So bottom line, the only solution to the problem is experience. After a while, you will be able to "feel" where your plane is, and you will have mastered the art of keeping your plane visually "above" the tree line until you are sure you are past it. After a while, it becomes second nature.