P6 Actual Cost

Hello,

In my P6 current settings, the actual cost is calculated as Total Units * Rate. The total units are collected from the period units. Is there a way to collect the actual cost from the Period Actual Cost? Say, the actual cost in Jan is 100$, Feb - 150$, I want to see 250$ in the Actual Total Cost, regardless of what units are? What settings do I need to change?

If this question has already been discussed, please refer me to the relevant thread.

Thank you very much.