I use the custom column "Date Last Read" with a Day-of-Week, DD, Mmm, YYYY format.
If the date is blank, I have never read the book. Any real date shown is the last time I read the book. Knowing the date I last read a book is more important than a T/F answer because some books are more easily remembered. Despite that, beloved books can be read and re-read regardless of how little/much time has passed.
While the date is overkill, sometimes if I want to return to a book I looked at recently, I know it was a certain day of the week, not the exact date, so I can look for that day of the week. The format... well, I kind of like the European(?) standard more than the American one.
PS: I use two ratings columns. One is the standard calibre rating which gets filled by downloaded metadata, the second is my personal rating of the book. I like to see if I agree with the fan-base.
