If the fretted 12th fret note is sharp, adjusting the screw clockwise will improve intonation. Turning the screw anti-clockwise decreases the length of the guitar string. If the fretted 12th fret note is flat, adjusting the screw clockwise will improve intonation. Only make small adjustments to the saddle at a time.
How do you know if intonation is off?
When an instrument is properly intonated, all the open strings and every note on the fretboard sound at their correct pitches. If your guitar still sounds noticeably out of tune even after you've tuned the open strings, the intonation is off—that is, the instrument is out of tune with itself.