How do I know the 90-day rule is logged accurately?

Q I make frequent visits to Portugal and therefore I am acutely aware of the 90/180-day rule for British travellers, limiting our freedom since Brexit. But I am concerned that there seems to be a dangerous anomaly in the official calculation of this rule. I keep my own spreadsheet of time spent in the European Union, taking into account past trips and future plans. I also use SchengenVisaInfo.com, which seems to be the official site. But I have put all my travel dates into the website and there is an 11-day discrepancy – with the website apparently excluding a trip I made at the start of December 2022.

If this is a widespread problem it could result in UK citizens overstaying the 90-day limit, with possibly dire consequences. Can you give this the serious and urgent consideration it appears to deserve?

John P