The default calculation Add calculated > Statistical > Average works for row average. In your report, it might give you an average of total days flagged impediment for resolved issues per week over time.
If you would like to calculate the average from issues, you would like to use a custom calculation. The best option would be to calculate this by dividing the total days with the count of issues that were flagged.
You can use a hidden measure Issues with days flagged impediment resolved for this.
CASE WHEN
[Measures].[Days flagged impediment resolved] > 0
THEN
[Measures].[Days flagged impediment resolved]
/
[Measures].[Issues with days flagged impediment resolved]
END
Please use autofill and reapply any measure in the formula to avoid some typo errors.