How are years of experience counted for determining level on salary scale for a Netherlands postdoc?
I'm post-doc'ing at CWI now; it's not a university but I'm sure it's the same pay scale - I'm also on scale 10. What I can tell you is:


Your slot/rank on the scale is, AFAICT and as @Niko suggests, equal to SOME_CONSTANT + calculated years of experience after the conclusion of your PhD. It doesn't matter what happened before you concluded your PhD. However, I got started on level 9, having only 3 years of calculated experience. So I can't verify that SOME_CONSTANT = 5. Or maybe it's an NWO-vs-universities thing.
The calculation of the number of years of experience is to some extent negotiable. In my case, I got my 3 years in industry after my PhD counted as 3 years of experience; and it was not clear from the get-go that this would indeed be the case. 


Finally, I've not actually read it, but perhaps the collective employment agreement for Dutch universities has the official wording on this matter. (I've linked to the 2015-2016 version but I don't think this point has changed.)