I expect you're visualising a kind of metric similar to
a Cumulative Distribution Function (CDF for short).
In Tableau it is usually a Table Calculation in a form of
RUNNING_SUM( [some_aggregate] ) / TOTAL( [some_aggregate] )
And you'd like to get (as a Table Calculation, of course)
a minimum between your (sorted / ranked) Dimension values
where the CDF value is approaching ( >=) 80%.
Please find the attached workbook (version 10.2) as an example.
Hope it could help a bit.
CDF_v10.2_YF.twbx 2.3 MB
Philipp, you're welcome.
Greetings from Moscow :-)