How can I create a "Calculated Field" in CRM?

How can I create a "Calculated Field" in CRM?

In order to calculate "Profit" analysis; I need to add a calculated field to the "Potential". What I had done is I had added a % field called Profitability(%). Then I need :

a) Either a calculated field which multiplies "Expected Revenue" with "Profitability(%)"
or
b) A report field that will be able to calculate this.

Any suggestions?
Regards,
Haluk