Here's my take on it.
The pickup/s should be adjusted in the play position...seated with the Bass on your lap. Use a 6 inch steel rule. While depressing the E (or B)string at the last fret, measure the distance from the bottom of that string to the top of the pickup magnets for that string, both front and rear. Adjust for a distance of 4/32 inches. Do the same with the G string but, this time, aim for a distance of 3/32 inches. Final adjustment may have to be made to the G side by ear, so that the volume is even across all strings.
The above procedure applies to all pickups, if more than one.
The Ernie Ball method (for a one pickup Bass) is basically the same EXCEPT the procedure
doesn't involve depressing strings at the last fret. Still in the play position, the height for the E (or B) string is 5/32 inches and for the G it's 4/32 inches. Again, the G may have to be adjusted for even volume ascross all strings.
After adjusting to EB's method, you'll find that the distances are exactly the same is in the first method.
The measurements are designed so that the magnets don't interfer with the string's vibrations should you venture up to the last fret while playing. You should be if you're practising your modal scales
After adjustments, if you have two pickups, the front one (near the neck) is going to be lower than the rear (near the bridge).
Also, the pickup/s will tilt down slightly on the neck side due to the fact that the strings slope gently from the bridge saddles to where they meet the frets on the neck.
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