How do I get the sum of column as a % of the sum of another column in a summary report?

You would need to upload a table with a concatenated field of the

Category Listing, the Spend Owner and the Month.  That field would be set to be the Key field of the table.


so like
Guaranted-TD-201801 
Guaranted-TD-201802
Guaranted-TD-201803
.
.
Guaranted-MRO-201801
Guaranted-MRO-201802
Guaranted-MRO-201803



Then  on the details table you would create that same field by formula and then use that field in a relationship to do the summaries of the total spend and total savings.

Then calculate your % up on the summary record.

Then look that % savings down to the details table.

Then shows that field in the summary report as an "Average".  Note that the left most column of the report for the grand total average will be not mathematically correct  and you will not be able to suppress it from the report.