Avg Turn Around Time Tableau Calculation

Hi, 

I am trying to define a a formula in tableau.  

1. Avg Turn Around Time for Approval (in days) 

2. Median Turn around Time for Approval (in days) 

I have a approval criteria defined already  

COUNT(  

IF [Status] = 'Approved'   

   AND CONTAINS(LOWER([Comments]), 'approved')   

   AND CONTAINS([Subject], 'RESIS Approval')   

   AND {fixed [Document Number],[Document Scope],[Assigned To: Full Name],[Latest Version: Full Title],[Latest Version: Publication Lead Function],[Latest Version: Status],[Subject],[Status]:count([Comments])} <= 1  

THEN [Document Number]   

END ) 

But i want to calculate these fields based on my approval criteri only , this is not about the date diff of created date and completed date  

The turn around time for approval based on my approval condition  

1. Avg Turn Around Time for Approval (in days) 

2. Median Turn around Time for Approval (in days) 

Thanks and appreciated your help.