"Follow up question - is the figure different for the neck and headstock, as compared to the body? I'm not in the habit of keeping my guitars in cases but realistically I suppose in-case humidification in the only way to get any moisture into the neck. I am thinking of getting a soundhole humidifier or more likely a dampit type humidifier and leaving it in overnight - once a week would probably be enough where I am now - but it won't do anything for the neck."
There is no different "figure" for the neck and headstock because you are still measuring the wrong value. You want to know the moisture content of the woods, not of the surrounding air.
Since that value cannot be easily measured, for the average guitar owner it is safe to assume whatever safe R.H. value is surrounding the body is also sufficient for the neck and headstock.
Of course, that very statement says we still aren't measuring the moisture content of the body either, so we have a bit of a decision to make. Thus, the +/- 20% range
given for the measured R.H in the room and at a location close to your guitar(s). Those are target values, not absolutes. Though, if the body of the guitar sits in a R.H. between 40 to 60% R.H,, then you could assume the neck is also at a value close enough for the safety of your instrument. Where the body goes, the neck must follow so ... (more on this later)
Consistency is far more important than just moisture.
Consistent temperature and R.H. are needed to keep your guitar in top shape. If you've read this thread, you should catch on to the fact R.H. is a constantly fluctuating value which moves up and down every time your HVAC system runs. By changing the temperature of the air surrounding your guitar, you are also affecting the R.H. value. In case you didn't realize how an air conditioner operates, a technician will check your vents to make sure the air blowing into the room is approximately 20 degrees cooler than the existing air in the room. The opposite is generally true for the furnace. However, both systems create forced air entering the room that is less moist than the existing air.
Therefore, the most important thing you can do for your guitar is to place it in the room as far away from the HVAC vents (and the outside walls and windows) as possible to ensure consistency.
The moisture content of the woods is always seeking equilibrium
with the R.H. value in the surrounding air, which in most forced air systems is always entering the room slightly less moist than the air within the room. If you are aware of the frequency and length of time your HVAC system runs, then you should have a good idea how often you should check your hygrometer. Just don't get overly concerned with any one reading, consistency within the desired range over time is the key.
"The soundhole systems (the ones with a cover) create an enclosed space in the body of the guitar so you aren't losing all the moisture to the environment - but this means that the rest of the guitar doesn't get any love. I am probably overthinking it at this point and will just get a dampit and not worry unless the action suddenly changes. I don't suppose there is any harm in running a damp cloth up the back of the neck every few days - there is not really any glue around there and it won't get wet enough to go moul
Consider the various woods used in a guitar and the amount of exposed grain. More dense woods tend to have a higher oil content which acts to keep the material at a proper moisture value. Spruce, rosewood, cedar and mahogany all have different oil contents and, therefore, all require a different preferred R.H. value to maintain their "seasoned" tonal qualities. You cannot be concerned with each and every small area of your guitar when it comes to moisture, which is why you have a broad range of R.H. values to work from.
While it may cause no harm to your guitar to run a "damp" cloth up the neck, it will certain do nothing to protect the neck in terms of moisture content. All points on the neck other than the exposed end grain at the body/neck joint are sealed with some form of protective material. Even if this were not the case, the moisture from the cloth would only be penetrating the top few tenths of an inch into the wood of the neck. That's certainly not going to change the neck wood's total moisture content.
It was mentioned that there isn't much to concern yourself with unless the action of the neck rapidly changes. That too is missing the point. Neck angle is different from neck relief yet both affect the height of the strings above the neck. Of course, the cut of the string slots in the nut and the height of the saddle above the bridge also add to the action of the guitar. Assuming the nut and saddle are set comfortably at this time, then it is the neck angle which is most commonly the cause of a rising action.
However, most people mistake what is happening with the neck as being only neck issues. If the neck is not bowing or warping, then the relative neck angle is first affected by changes in the body materials.
If the lower bout begins to belly, the bridge is likely to lift which will raise the saddle and the angle the saddle strikes the strings. Likewise, if the bout begins to sink, the saddle must follow. Movement in either direction will affect the action of the strings, and the intonation of the guitar, due to the changes in the neck angle relative to these parts of the body. If the goemetric changes have become permanent, many techs will suggest shaving the saddle or the nut slots to return the proper string height above the fretboard. With this sort of quick fix, the trade off is against proper intonation. Eventually, the saddle has no more adjustment available and then a neck reset is the most common repair.
This is NOT the proper technique for achieving a neck reset; https://www.youtube.com/watch?v=K1H4xy52RuM
If the upper bout begins to sink, the sides and back of the guitar typically follow the movement of the top, which is likely to change the physical length of the guitar. If the back flattens, the same dimensional changes will occur. All of these changes in the geometry of the guitar's body will ultimately affect the guitar's neck angle. Small changes in geometry occur almost constantly and are not to be considered a danger to the guitar any more than the fact your all wood table is changing by 0.05" should bother you.
However, if the neck angle changes dramatically or if the bridge begins to lift from its rear point connection to the body, then you should have the guitar attended to by a qualified technician. Loose or broken braces may also account for these same changes though the most likely cause is neglecting the consistency of the temperature and humidity values surrounding your guitar. Glues deteriorate in extreme heat and cold as well as moist and dry conditions so don't leave your guitar in your car or in a damp, cool basement/hot, dry attic.
In case you haven't figured out exactly what changes the neck angle, it is not the moisture content of the woods in the neck itself. The neck follows changes in the body geometry
. Therefore, concerning yourself with moisturizing the neck is missing the point. Keep the guitar in a R.H. which is consistent, not too high nor too low for days on end, and your guitar will be happy. Make changes to the temperature and the moisture content of the surrounding air slowly thus allowing your guitar to acclimate to its new environment. You have a broad +/- 20% value to work with and the guitar will likely survive much higher and much lower values for days before the thin woods of the body begin to change.
Play your guitar regularly and become accustomed to the tonal qualities of your guitar when it is slightly moist and slightly dry. Use your ears to detect problems before they become serious. That's the "love" your guitar needs.