Your Formula is CORRECT, What wrong you are getting, Can you attach a workbook to help u faster?
Since the FIXED calculation is performed on the underlying unfiltered table, not the data showing in the visualization, you probably have rows of data filtered out of your visualization that are impacting the Average value.
If this is correct, then you will need to use nested LOD statements. FIXED and EXCLUDE. This will allow you to also exclude items from the average calculation. Nested LOD Expressions
One More thing, since you are using FIXED, Ypu need to add your Filters to CONTEXT