Customer Segmentation By Dynamic Percentages

Hello,

I am struggling to perform a correct segmentation of customers based on their [Total Revenue] as follows:

First 20% of them = Big customers
Next 30% of them = Medium customers
Last 50% of them = Small customers

I have only a fact table where I created a calculated column for rank (based on total sales per customer for all years):

Rank = 
RANKX(
    ALL('Fact'),
    CALCULATE(
        SUM('Fact'[Total]),
        ALLEXCEPT('Fact',
            'Fact'[Customer Number]
        )
    ),,
    DESC,
    Dense
)

Having only 10 distinct customers, the segmentation should be:
First 2 ranked = Big Customers
Next 3 ranked = Medium Customers
Last 5 ranked = Small customers

However I don't know how to dynamically peform this in a calculated column in the fact table (I only hardcoded it) so I use the column in a matrix like this (for example):

+ Big Customers              Total Revenue
       Customer 1                   xxx
       Customer 2                   xxx
+ Medium Customers
      Customer 4                    yyy
      Customer 5                    yyy
      Customer 6                    yyy
+ Small Customers:
      Customer 3                    zzz
      Customer 7                    zzz
      Customer 8                    zzz
      Customer 9                    zzz
      Customer 10                  zzz