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

At present, Quick Base does not support math operations against columns on a Summary report, although the question does come up pretty often on this forum.

Depending on your table structure it is sometimes possible to get around this limitation with another table where the data can be summarized up to, but I see in your case, that other table would need to have every possible combination of the Category Listing, the Spend Owner and the Month.