Using that single record as an example, a simple calc of:
SUM([Amount B])/SUM([Amount A])
Will provide the .30 value you're looking for. Does your raw data include a column for Customer, Amount A, and Amount B? If not, could you post a sample packaged workbook using some randomized data?
If your Amounts are stored in one column together, you may need to separate them out using a calculated field -- but I'd need to see an example of your raw data to fully understand what your original data source looks like.