Too me, the most ergonomic angle for the monitor is such that the center of the monitor is facing directly toward your eyes. While standing in front of the cab like you would be when playing, make an imaginary line from the center of your eyes to the center of the monitor screen. The plane of the monitor screen should be perpendicular to that line. So the tilt of the monitor would be determined by the height of the monitor relative to the height of your eyes. The lower the monitor is in the cab, the more it needs to be tilted back.
Of course, even with this "perpendicular eyeline" rule, you still have to decide how high you want the monitor to be relative to your eyes. When standing over the controls to play, I think having my eyes look about 40 degrees down from straight ahead is comfortable. This means the monitor would be tilted back 40 degrees from vertical, and at an appropriate height lower than my eyes.