1) Take total of revenue :
I.e In my case Total = 5500
2) Now create a variable
Limit = 80% of 5500 (In my case it would be 4400)
3) create a new report ,
Create one more variable for running sum & sort the report by revenue.
drag following columns
Customer no , revenue , running sum.
4) Apply report level filter , where running sum <= Limit.
Done!
Thanks,
SB