80 is 'Recent'
Boy do you read fast.
If you want to track your reading, you might consider adding a Custom (Date-time) column.
Since it is a Timestamp column, it sorts down to the second, so you know the exact order marked
But there is MORE
If you use Calibre Companion (on android), it has the ability to sync (wireless connection) a
Read date (or Yes-No, if you set that kind up)