This can be done a couple of ways, the first is to create a calculated field that uses a table calculation similar to the following:
LOOKUP(ZN(SUM([Sales)), -1)/ ZN(SUM([Sales]))
Place this on the view, right click and select Compute Using-->Table(Across).
Another option is to create 2 calculated fields, one for each Product Name.
if [Product Name]='Books' then [Sales] end
if [Product Name]='Computers' then [Sales] end
Then, create a calculated field that uses these two calculations:
sum([Books Calculation])/sum([Computers Calculation])
Using this example, Product Name will need to be removed from the Columns shelf, and these three calculations should replace it.
Hope this helps!