Recommended pickup height for a Stratocaster is 3/32" (2.4mm) from the top of the pole to the bottom of the string on both the low E and high E strings while the strings are pressed down at the highest fret.
After adjusting the pickup height to 3/32" (2.4mm), use your ear to fine-tune the heights of the pickups to achieve a balanced volume across all positions.
3/32" is a general rule, sometimes a half turn closer or further will dial in a "sweet spot" for your particular guitar, pickups and string gauge.
In general, closer to the string will yeild greater output, but too close to the string will choke the string vibration and sustain.
Farther from the string can reduce output, but increase clarity.
Experiment until you find the best pickup height for you!
Here is a blog post that explains in greater detail: Adjusting Pickup Heights
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