hi @Hendrik_TnB ,
I guess one of your clients need to have this calculated 
in two steps you have the solution. It starts with filtering out the on the Acc Name, in my example the article. This list of items FormulaMap() to get the delta
The second step is to show per row the value already see in this list of items using Slice() and Last() with an other filter that gives you the index per group of alike articles.
Let me know how it goes and if this makes sense to you.
Cheers Christiaan