As most people have said, its a 'rolling' 180 days. Although I have a passport from a Schengen country, mrs t does not so we have made a spreadsheet which shows us exactly what the 'current count' is. I recommend you do this (its very simple but if you want mine, PM me and I can send it to you).

I would not try to assume your visit is not being 'watched'. Your passport wil get scanned as you enter the Schengen area and it goes into some big database. Now I understand that the cross-country sharing of this database is not 100% operative yet, but it will get better and better. It would be very easy to build in some algorithm which triggers an overstay which will cause hassles when you leave, but more importantly: cause hassles when next you want a Schengen visa (and if you dont need one: hassles when you next try to enter). Its not worth it.

(and if the one-day CPH visit had anything to do with the Schengen rule story, then dont bother, it just costs money!)