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I'm sure a lot of you are familiar with this, but for those who have never adjusted a neck before, let me clarify two terms first: “relief” and “action.” When I say “relief,” I'm referring to how much the neck bows in the middle. Because of the string tension, the guitar neck isn't perfectly straight. It has a little dip in the middle—think of an archer's bow as an extreme example of string tension pulling two ends toward each other. “Action” is the distance between the string and the frets. The higher the action, the higher the string is suspended above the frets. Relief and action are matters of preference, but what most people look for is the optimal balance of relief and action that lets you comfortably employ your particular set of playing techniques, while getting the tones you want.
The idea of looking from the headstock down is to sight the edge of the neck in relation to the straight pull of the low E string and see how much relief the neck has in it. There's nothing wrong with this method, but the mind and the eye can play tricks on you. Sometimes you look at the neck and it appears to have the right amount of relief but it still just doesn't feel right.
Here’s an alternative to the sight method that eliminates this discrepancy. Get a feeler gauge and a straightedge that is roughly the same length as the guitar neck—both items can be found at a hardware store. With the guitar tuned to pitch and in playing position (not flat on your lap or on a bench), set your straightedge on the neck, resting across the frets between the low E and the A strings. Personally, I set my neck at .006" at the 8th fret, so for my guitar, I'll use the .006 gauge and slide it in between the straight edge and the 8th fret on the low E side of the neck. It should slide in with just the slightest bit of resistance. I'll be able to feel the fret and straight edge with no wiggle room between them. This lets me know that my neck is where it should be.
If there is too much resistance or the gauge won't slide in at all, I'll adjust my truss rod by slightly turning it to the left (counterclockwise) to loosen it, opening that gap so my feeler gauge will slide in easily. If the there’s very little to no resistance when I insert the feeler gauge, I know I need to tighten the truss rod by turning it to the right (clockwise), which will straighten the neck. Mathematically, this method puts your guitar neck in the same place every time.
If you travel with your guitar or are a touring musician, be sure to let your instrument acclimate to the room you're in before you attempt to adjust the neck. Varying temperatures and humidity levels from place to place are the reasons we have to make these adjustments in the first place. Often, issues work themselves out just by leaving the guitar in its case to warm up after being brought in from the cold. While touring during the winter months, I rarely touch a guitar for the first couple of hours of it’s coming off the truck. It may be cold outside, but remember, that's where the wood in that guitar “grew up.” It's not going to hurt it. The damage comes from how quickly it heats back up.