Assuming you have built a table with County A,B,C down on the side, and QTR 1,2,3,4 across the top, and % of Total for the statistics at the top. By default, the % of total is based on the whole table. If for each county, if you want to see the % of total for each QTR, take the QTR label and drop onto the % of Total heading. What you get is the % of total based on the sum of all the QTR for each county. You will see this better if you drop the key word "All" onto the heading QTR, then you will see that the % sum to 100 for the last column. If on the other hand, you want to see the % of total for each county given a quarter, click and drag the label "County" and drop it onto "% of Total". Now, the denominator is based on the sum of the count for all the counties.
Again, to see it clearer, just take "All" and drop onto County.