I'm having trouble understanding coordinate space using perspective in OpenGL. Basically I can't figure out how many units will represent the entire screen given a specific z-value or how spacing should be done, etc. In all the tutorials I've seen they plot these nice shapes placed evenly apart in different areas of the screen x units back in the z-plane but I have no idea how to inherently understand how many units I have to work with at that z-value and how to space things, place them, etc. Is there a good primer on this or some formula using something like the fov angle, viewport size and camera position to figure these things out? Recently I was asked to plot two dots in perspective 1 inch apart on the screen and I'm having trouble figuring out how to achieve this (1280x800 10" monitor display).