Hey Thomas,
I was going to say the default value for porosity is 0.3, but I'm not sure any more. I used one of my small examples and ran particles with the default settings, then set the porosity explicitly under "SET" to 0.3 (and 0.6 / 0.15). A porosity of 0.3 returned very similar results, but not quite the same. You might wanna try this yourself ... I'd say 0.3 is a close enough assumption though.
Cheers, Chris