The best screen position is perpendicular to the nearest window. Other angles leave you with strong brightness differences between the screen and its surroundings, which can easily cause vision problems. If you have no choice, at least cover the window with a neutral color shade. Bright reflections in the screen will disrupt your vision and eventually cause eyestrain. To test for these, hold a small mirror in various positions on the screen.
Reflective glare shows up as very bright light coming from the mirror.
Reposition the screen or reflective objects to reduce the problem. An optical quality glass anti-glare filter may also help. Setting the screen to display dark characters on light backgrounds will reduce the chances of eyestrain. Veiling glare or discomfort glare can occur when too much light comes from above. Check for them by shielding first your screen, and then your eyes, from ceiling light with a piece of paper.