I have a facttable and a dimension table. I would like to show as a measurement the subtraction of to members of the dimension table. How can i do this?

I first tried to put the results of the calculation in the facttable but this did not work very well cause the row appears in the fact table more than once, so that when the cube aggregates the numbers are added more than once and are inaccurate.

I cannot put the results of the calculation in the dimension table because they are added multiple times and should only be one.

sorry about the multiple posts... the forum gave me errors like it did not post so i tried again.