Quartile Totals

I have a dashboard based on multiple published Cloud data sources - 

= Agent Daily Structure 

multiple KPI data sources on staff number and date only 

The end user wants to see multiple KPIs on a dashboard page showing the grand total and then splitting the agents into quartiles (just the 4 totals showing).  They then want to be able to use filters to change the month or drill down/by department/manager etc to which the quartiles will re-adjust. 

I tried building my initial data source in Prep, as all of the sources are already published to Cloud, with left joins on staff number and date to the primary data source and cleaning up, rather than the 'blends' but the agent-level totals for the KPIs did not reconcile against the data source in isolation, before even trying quartile calculations, hence trying it the 'blended way'.  This is giving correct agent and grand totals. 

I have tried various ChatGPT suggestions but none of them work (sometimes its a great AI tool, but in this case no!).  I've not been able to find anything in forum posts/guides that seems to suit my scenario.   Can you help?  

=  

{ FIXED [Agent ID] , [DateTrunc - Month]: (AVG([Evaluation Score])/100) } 

=  

IF [Fixed LOD - QA] <= { PERCENTILE([Fixed LOD - QA], 0.25) } THEN "Q4 - Bottom Performers" 

    ELSEIF [Fixed LOD - QA] <= { PERCENTILE([Fixed LOD - QA], 0.50) } THEN "Q3 - Coaching Needs" 

    ELSEIF [Fixed LOD - QA] <= { PERCENTILE([Fixed LOD - QA], 0.75) } THEN "Q2 - Good Performers" 

    ELSEIF [Fixed LOD - QA] > { PERCENTILE([Fixed LOD - QA], 0.75) } THEN "Q1 - Top Performers" 

    END  

Quartile groups on rows, QA score on column and the text mark to display in columns.  This works 90% for QA score (the totals for each Q at least look reasonable and adjust when filtering) - the odd agent appears under the wrong quartile when adding agents to the view to check the quartile calc/sorting highest to lowest.  But if I try the same calc on another Quality metric in the same data source it doesn't work at all - its grouped everyone under the bottom 2 quartiles. 

I've tried multiple ChatGPT suggestions using Windows_Percentile and Rank_Percentile and using the staff number or staff number and date on the 'detail mark' and keeping the first row, but none of these work. 

I need a reliable way to split agent performance on a KPI into quartiles using this blended data source, that can be filtered.  Surely something like it exists/its not an impossible ask!!  TIA