Originally posted by ArchAngel
Eric -
The problem with your thesis is that your assuming non-standardized chart geometry.
As you observed, you can't just lay a protractor on different screens and get the same results.
To uniformally apply a chart angle across different chart scalings you need to compute it from a standardized price/time grid (i.e., X units of time per Y units of price) and then draw the angle lines based on that grid (e.g., 45 degrees is 1 grid box horizontally and 1 grid box vertically). Some software does this with what's called a Gann Box to lay out the basic grid.