Unfortunately, calculated fields caculate off the original table, not the
pivot table.

An alternate option is to calculate outside the table. So, assuming you have
your headers in A3 through C3, and each subject takes up to rows, Try going
to D5 and typing C5/C4. Highlight D4 to D5, copy, paste down  to the bottom.

The calculated fields always use the Sum of the pivot fields, even if
you're using another summary function, such as Count. For text fields,
the sum will always be zero, so that's why the calculation isn't working
the way you want it to.

In your source table, you can add two columns -- one to count the
traffic types (TraffCount), and one to count the sales dollars
(SaleCount). You could manually enter a 1 in each column, or use a
formula to calculate the count.

For example, if the Sales Dollars are entered in column G:
  =IF(G2=0,0,1)
Do the same for Traffic Types, or if there will always be a Traffic Type
 entered, just type a 1 in the Traffic Count column.

Then, in the pivot table, create a calculated field with the formula:
  =SaleCount/TraffCount
and format it as percentage.