Fender Stratocasters feature adjustable string saddles on the bridge to set the string action, also called string height. Action is highly subjective, and there is no such thing as the "perfect" string height to anyone but the individual. Stratocasters and guitars with similar bridges have an advantage over side-adjustment bridges, as the individual strings can be adjusted to player preference. There is also an increased margin of error because of the rounded contour of the neck's fretboard. Make the adjustments slowly and methodically for best results.
Measure the gap between the bottom of the string and crown (top) of the 17th fret with a ruler. Depending on the neck radius, the measurement on the bass side (three lower strings) should be approximately 4/64 inch (1.6 mm) to 5/64 inch (2 mm). The treble side (three top strings) should be around 4/64 inch (1.6 mm) to 3/64 inch (1.2 mm).
Insert the hex wrench into the hex screw on one side of the lowest string (E or 6th) saddle and adjust up (turn clockwise) or down (turn counter-clockwise) until the string reaches the target measurement on the 17th fret. Adjust the second hex screw on the saddle to level and stabilize the saddle.
Continue adjusting the saddles of the remaining strings until all target 17th fret heights are reached.
Play the guitar and evaluate the action.
Raise or lower each saddle as in steps 1 through 3 using a higher or lower target measurement to adjust the action to your liking.
The string contour is preset and the saddles are not individually adjustable for the tremolo bridge on some Strats. Adjust the two screws on either side of the bridge to raise or lower string height.
Since adjusting the guitar neck can overall affect string action, it may require adjustment prior to setting string height with the saddles.
Intonation may be affected by adjusting the saddles, and should be checked and adjusted if necessary after setting the neck and string action.
Adjusting the strings too low may cause them to become too close to the top of the pickups and cause a bad tone. Adjust the pickups via the adjustment screw on either side of each pickup after adjusting the action to cure the problem.
Adjusting the string action too low may cause buzzing frets. Fret issues must be entrusted to a professional, as attempting to file or sand frets can cause expensive damage to your instrument.
Making multiple adjustments without experience may result in an unplayable instrument and a more expensive set-up bill at the repair shop.