Custom Grouped Aggregation

In a table, when grouping by columns, I can select an aggregation type such as sum, which conveniently gives me a total for that group in the "grouped row". However, if I have a column with custom calculations, ex. column c / column d, the grouped row aggregation is calculated incorrectly, no matter what type of aggregation I select. If I don't select an aggregation, nothing shows up. How do I get the grouped row to show the correct calculated aggregated total when columns are grouped?

Ex Table

Account Campaign Visitors Leads Leads / Visit
GROUPED of A 100 10 16% (Should be 10%)
A A 25 1 1/ 25 = 4%
A B 75 9 9 / 75 = 12%