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

So if i had a duplicate table to the main one with all the same fields in it I could potentially get the % figure.  I assume that would be by adding a summary field in the master table relationship to the second table?  That is entirely possible because the data come from an upload and would not be too difficult.  

How would I set that up and how would I incorporate it into my reports?