180 day limit

It’s a rolling 180 days - count back from current date 180 days and see how many days in that period you’ve spent in the Schengen.  Each new day you count back 180, so eventually days spent in the Schengen will start “dropping off”.  The best, easiest, safest and most reliable thing to do is use one of the various free online Schengen calculators that will tell you how many days you have left in the Schengen.

At 189€ per day fine for each day overstayed, you really don’t want to run the risk of miscalculating and overstaying.

I downloaded a Schengen zone tracker onto my phone. Each time you plan a trip, just put the dates of entry and exit in and it calculates your remaining allowance. As you come and go the allowance available goes up and down as the 90 days are a rolling amount. On mine it's shown as available days on each date.

To be fair until they sort out the electronic gate system to track your comings and goings, rather than relying on a passport stamp, it's very unlikely that they would pick up a day or 2 in the wrong place. I would take hours going through a regular travellers passport to work it all out. The problem would be if you were in a traffic accident or something like that and the Guardia checked on your status. Then you could be fined or even barred from returning for a period of time.