How to perform a function only for a unique value from a column

You could add an index column that restarts with each new NUMBER, and then add a new column with your formula conditional on the index.

See Create Row Number for Each Group in Power BI using Power Query - RADACAD.

Steps would include grouping by NUMBER, keeping all rows, adding an index column, then expanding.

Then you could just add your column with the Custom Formula option in the UI.

Hi,

i was working on the same schema as karen578

this is the transformations in power query

let
Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjQyNlHSUTIw0TMw1zMyAjINDeFMXTOlWB3i1BgAAZoaBFPXEqzGEghQ1RhYINSYUk9NLAA=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [NUMBER = _t, PLANNEDDATE = _t, REALDATE = _t, BACKLOG = _t]),
#"Grouped Rows" = Table.Group(Source, {"NUMBER"}, {{"AllRows", each _, type table [NUMBER=nullable text, PLANNEDDATE=nullable text, REALDATE=nullable text, BACKLOG=nullable text]}}),
#"Added Custom" = Table.AddColumn(#"Grouped Rows", "Custom", each Table.AddIndexColumn([AllRows],"Index",1,1)),
#"Removed Other Columns" = Table.SelectColumns(#"Added Custom",{"Custom"}),
#"Expanded Custom" = Table.ExpandTableColumn(#"Removed Other Columns", "Custom", {"NUMBER", "PLANNEDDATE", "REALDATE", "BACKLOG", "Index"}, {"NUMBER", "PLANNEDDATE", "REALDATE", "BACKLOG", "Index"}),
#"Changed Type" = Table.TransformColumnTypes(#"Expanded Custom",{{"NUMBER", type text}, {"PLANNEDDATE", type date}, {"REALDATE", type date}, {"BACKLOG", Int64.Type}, {"Index", Int64.Type}})
in
#"Changed Type"

and finally the dax