vacuum reading on idle

[Bean]

Hi,

for quite some time I got the feeling my engine is starting to wear a bit more than usual and although it's still driving fine and going hard I do have the feeling that something is a little off...

my engine is still completly stock and I don't see the point of changing that as long as I have the feeling something is off, as every bit of tuning would always be masked by something I haven't yet diagnosed.

So to the point

I hooked up a sensor lately to read the vacuum of my 4A-GE @idle (900rpm) and 'only' got ~40kPa (which equals ~11.8inHg)

in theory a good value should be around 30kPa or 20inHg - which is quite a bit off from my readings - though different engine get different values...

so does anyone have numbers to compare to or the option to test this aswell (easily done with a vacuum or boostgauge, as these mostly show vacuum as well)?
engine obviously warmed up and in operating temparature

Cheers Bean

PS: I do have some ideas what's wrong but would like others input first before I go there as I currently lack the tools to further analyze the problem