We see more visual field below the horizon than above this, so at this position the user should comfortably be able to see more of the screen. At this distance you should be able to see the viewing area of the monitor without making head movements. If text looks too small then either use a larger font or magnify the screen image in the software rather than sitting closer to the monitor. If any adjustments feel uncomfortable then change them until the arrangement feels more comfortable. Good posture is the basis of good workstation ergonomics.
Good posture is the best way to avoid a computer-related injury.
Make sure that the upper arm and elbow are as close to the body and as relaxed as possible for mouse use - avoid overreaching. Also make sure that the wrist is as straight as possible when the mouse is being used. Make sure the user sits back in the chair and has good back support. Also check that the feet can be placed flat on the floor or on a footrest. Make sure the head and neck are as straight as possible.