And for fun, here is an action chain that does the job using the above template. It sets the column #my_float to the highest series index for the book's series. Import the chain then change the Single Field Edit to use your column. There is probably a way to make it not flash the selection, but I'm not enough of an action chains guru to know what it is.
It isn't the fastest thing in the universe, but given it is run on demand I think it is good enough. It would be much faster if it used a "Python code" action that operated on all books in the library instead of book-by-book. This would also get rid of the flash. I leave that as an exercise for the reader.
