ORIGINAL: britbrat
Our runway is 200' long, so I try to visualize the number of runway lengths (or fractions) above ground. Crude, but it works (I think).
Human eye has different depth perception in the horizontal and vertical planes. Objects 150' high seem much far away than 150' away in the same horizontal plane. So a plane visualized to be one runway length (200') high might be at 80' or who knows how high.