There is no way to identify the “last row” in a table without requesting all of the rows in the table or view. This is because different views of the same table can include different records in different orders. 

In order to update a cell value, you need to know the record ID of the record, not just it’s relative position. 

if you add an autonumber field and then sort by that field descending, then the first record would be the latest added record. This would need 2 calls to the api though.