Some random thoughts:

The right action height is as low as you can have it and not have the strings buzz, when played in your particular style. A guitar set up right for bluegrass rhythm would be useless for subtle, quiet fingerstyle, and vice versa.

Action height is a function of:

- saddle height above the top surface,
- nut height (and depth of nut slots),
- angle of the neck set (join of neck and body),
- amount of neck relief (curve in the neck - there should be a teeny bit),
- amount of "belly" (or, hopefully not, sinking)of the top - again, there should be a teeny bit of bellying up),
- gauge of strings used, and whether standard tuning is used, or one where some strings are tuned lower.
- and maybe other factors I don't even know about.

So adjusting the action is often a professional operation. An amateur can sand down or shim the saddle, or mess with the truss rod, but that's about it. And it's easy to make things worse.

Note that the bellying or sinking of the top varies with dryness or dampness of the wood, and therefore with humidity in the atmosphere. If humidity varies, the action may go up or down on its own.

If you live where humidity goes up and down a lot, it's a good idea to have spare saddles in different heights.

P.