Minimum datediff per unique value

Let's say i've got a couple of relevant columns: 

Each case consists of a number of documents (could be 1 could be 100 etc). For each doc number i have a Date last used.

I calculated the difference between the last time a document was used and today.

The problem is: I want a column added where for each case (so A, B and C etc) i see what the datediff is for the LAST used document in that case. How do I do this? 

Edit: Explaination what i want with it:

It might help if you know why I want to do this: I want to generate a visualisation table with al cases where the datediff of the last used docname is bigger than a certain number. For instance I want to see al the cases A t/m C where the datediff between last used date and today is 180 and all the cases D t/m Z where this datediff is 230. 
I made a column that looks Yes/no = IF([datediff between date last updated and today]>[column with 180 for case A-C and 230 for case D-Z], "Yes", "no"). 
This column i want to use to filter my page. I only want to see the cases where the Yes/no column gives Yes, because this are the cases where datedifference from the last used docname is larger than it should be.

BUT when i do it like this it gives me al the docnames within a case where the datediff surpasses 180 or 230 (for A and B). I don't want to know it per document, i want to know if the datediff of the LAST used docname in a case is higher than it should be..

For example: