Nako,
I have done a lot of measurements both inside and outside of my shop. The first day I got the meters (I bought one each of the 1 -5 and .5 - 2.5 meters) I read about 1600 in my shop. I had been doing a little work on my dust pickups for my jointer and planer but hadn't done much cutting of anything. Well, I thought my shop would be reading really good so that concerned me. Then I took the meter right outside the shop (this was the .5 - 2.5 meter) and I got readings of over 4500 so that told me that the ambient air quality inside the shop was better than outside. Anyway, since then the outside readings have varied from a low of about 400 to a high of over 5000. Of course in the spring with the pollen out, I would expect high outside numbers. Humidity (actually water droplets as found in fog) also raise the particle count dramatically.
At this point in time, I'm not as concerned with a particular number as I am with being able to see if the count goes up or down when I turn on my dust collector. And then to see if the count remains down when I start woodworking. Hope this helps.
Ed