Try this
some search terms.. | sort 0 - Field_Of_Interest | streamstats count as Rank | eventstats count as TotalRows | where Rank<(TotalRows * 0.7) | stats avg(Field_Of_Interest) as Required_Average
Try this
some search terms.. | sort 0 - Field_Of_Interest | streamstats count as Rank | eventstats count as TotalRows | where Rank<(TotalRows * 0.7) | stats avg(Field_Of_Interest) as Required_Average