from the syntax point of view I think you need an advanced aggr, maybe something like
Fractile( aggr(avg(sales), somefield),0.95)
this is from the Qlik help
fractile ([{set_expression}] [ distinct ] [ total [<fld {,fld}>] ] expression, fractile)
returns the aggregated fractile of expression iterated over the chart dimension(s).
The function argument expressions must not contain aggregation functions, unless these inner aggregations contain the total qualifier. For more advanced nested aggregations, please use the Advanced Aggregation function in combination with calculated dimensions, see Add calculated dimension... .
