Matthew, I think you're looking for this table calculation:
IF SUM([Values])=LOOKUP(SUM([Values]),-1) THEN PREVIOUS_VALUE(1 ) ELSE INDEX() END
It only works on sorted data. (See attached.)
Index Multiples.twbx.zip 17.2 KB
It works on unsorted data if you apply the sort in the table calculation!
I'll make some headway with this - the solution is helpful for a number of different applications so thank you.