identify rank number for topN% of a measure

[stsau]

I am testing pareto principle and calculating the top N% of revenue and the underlying responsible customers.
Customer are being ranked by amount of their rev value:

Revenue Rank =CALCULATE(

RANKX(ALL(RevData[enduser_cust_name]); [Total Revenue FY 18]; ;DESC);

ALLSELECTED(RevData[Portfolio])

)

Now I want to compare the numbers of all (unique) customers with the number of customers that make up N% of revenue, so I get the customers that are making up the N% of revenue:
( N% can be selected by What-If slicer 'Revenue %'[Revenue % Value] )

Top R% Customers =

IF(ISBLANK([Total Revenue FY 18]) ;

   BLANK();

     IF(RANKX(ALL(PRevData); [Total Revenue FY 18]; ;DESC) < (CALCULATE( [Unique Revenue        Customers];ALL(RevData) ) * ('Revenue %'[Revenue % Value]/100) );

[Total Revenue FY 18];

BLANK()

)

)

its all working ok, [Top R% customer] column is showing blanks, once selected N% revenue is reached.

However I am not able to get to identify the rank, that corresponds to [Top R% customer] becoming blank, which is the number of customers responsible for N% of revenue.
Searching the forum I see posts that relate to maxx(column), however this is a measure + its only a subset of the underlying table, so countrows would not work either.
Any idea appreciated.