Set the eye-to-screen distance at the distance that permits you to most easily focus on the screen. Usually this will be within an arm's length.
Set the height of the monitor so that the top of the screen is below eye level and the bottom of the screen can be read without a marked inclination of the head. Usually this means that the centre of the screen will need to be near shoulder height. Your eyes should be level with the tool bar.
People who wear bifocal or multi-focal lenses will need to get a balance between where they see out of their lenses and avoid too much neck flexing. The height of the monitor can be adjusted using a monitoor riser.