In the Data Model Editor, in addition to the measures you can pick directly from a Fact table, you can also calculate new measures on basis of the existing ones.
Right click the Calculations folder to add a new calculation:
In this example, Revenue per Unit is calculated from the Amount and Quantity measures. Double-click the measure names on the left to insert them in the calculations editor on the right.
In the TARGIT client, a Calculation will look like and can be used as any other measure: