When adjusting the neck of an acoustic guitar, what bending might be normal?

Recently I bought a second guitar, before I had for almost 8 years just one guitar and did not noticed that its action was quite high. When I took the guitars of others I often wondered that they where much more comfortable to play, but I thought it was an issue of the guitar itself and I had no idea how to adjust this. As I got my 2nd guitar I noticed the same, and also noticed that the saddle was much lower.

So I did some research and found out that replacing (or sanding) the saddle can help lowering the action, so I did this with my first guitar getting the action from 6mm down to ~4mm, see the picture:

But I am trying to get it even lower, but the saddle is already quite low. So I did some further research and found out that this could be achieved by adjusting the truss rod.

But before doing this, I would like to know. What bending of the neck is considered normal? I could not find any comparing pictures on the internet, so I am asking. My other guitar has also a slight bend, so I guess some curve in the neck might be normal, but I wonder what degree might be normal (so that I do not adjust my guitar wrongly)?

I also append a picture of the neck, but on it the bend is not as clearly visible as in real life:

So what bending is normal? Or should I aim for as flat as possible?

As written here:

Ideally we are adjusting the truss rod to render the neck as flat as possible without creating too much string buzz.

and they also write the optimal action is between .004-.006in (~0.127mm, see this sheet), so their might be plenty of room to adjust my action of 4mm.

• That 'optimal action' of 4 to 6 thou is the width of a human hair! If the guitar was adjusted so, the strings would not have room to vibrate. Taking the bottom E to be around 50 thou, you need at very least that same gap, at fret 12.
– Tim
Commented May 15, 2017 at 7:20
• :D Mh... 0.004" - 0.006" are the values written in the link; I am from Europe and do not eat Quarter Pounders with cheese^^ Maybe you can enlight me and write what .004" means then... Commented May 15, 2017 at 13:36
• The link is merely a conversion chart. Maybe the metric figures were wrong to start with. But human hair diameter is pretty well standard, at .005", and seems way too small a figure for the action measurement. Just seen the article, and it must be wrong.
– Tim
Commented May 15, 2017 at 13:50