Quartiling data range + adding slicers that filter column chart while maintaining quartiles

Hybrid Quartiles (Dynamic + Static) 

I have been trying to work through this problem for about a week and a half with no solution inside Power BI. I almost think that the program is incapable of calculating the desired result. However, I think all the smart people in this community may be able to help me out. 

Unfortunately I do not have the pbix file for this as it remains on my work computer and I do not have access to this forum on there. However, I have established a sample dataset and the outcome I desire through excel.
Please see attached: iCloud_OneDrive_QuartilingExperimenation 


The attached worksheet probably illustrates my problem the best so please have a look. Otherwise the summary is as follows: 
- I need to establish quartile factor values (used in a color coded legend of column chart) for a given range of sales data that are averaged for each customer. (over selected dates- slicer). 
- I then need to be able to filter that data by the account manager without it effecting the quartiles assigned to each customer. 

I have tried the ALLEXCEPT function (date)  to try and negate the effects the account manager slicer has on the quartiles assigned to each customer; this leads to innacurate results as the quartile cutoffs are calculated over the entire range of dates in the data and the quartiles come out wildly uneven. 
If there were a way to filter the ALL except function to only pull the selected dates instead of the entire range then i feel like the desired output can be accomplished - however i know this is not how this function works. 

Any Insight would be greatly appreciated.