I worked this out a while ago (and still have the spreadsheet). Currently it's 91, but I need another 25 or so rides of 100 miles to get it up to 100. I have done a lot of rides in the 90s without bothering to do the extra turn around town to get over 100.
Like CET, I've not worked out separate days on longer rides, as that would involve going through brevet cards and guessing, which hardly seems to fit with Eddington's concept. For example, I'm not counting rides before 2005, which is when I started to keep records.
It may not meet with the idea of a day's riding, but it is accurate, and consistent, and a bit of fun. Maybe I should only ride 300 km and above from now on...