What report and custom formula should I be using to calculate my Profit Margin percentage?

Resolution:
The Detail (Task-User) report can be used to create Profit Margin percentage in custom column.

The formula to calculate the profit margin is as following:  (Sell Price – Cost) / Sell Price * 100

The custom formula that can be used is: (BaseBillableAmount – BaseCostAmount) / BaseBillableAmount * 100

Related Items:

What are Custom Columns?
What are Custom Columns? Why might I use them?
How to Include Custom Columns to reports ?