Take last non value for next calculation

You don't need a PBIX file.  As I wrote, all you need to do is replace the lines in your own file after `#"Changed Type#` with those lines in the MCode I supplied.

Or just paste the code below into a blank query using the Advanced Editor in your PBI power query, and it will reproduce everything including the table.

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("ldBBCgIxDAXQu3Q9lv/TtJ1cw+3QhQsXgkvvj9UiyNgpFRJICw/ys22OBLKpW9z5+rjc7nUQAExWJ/BUSyCsD43r6vPr991lOcY577EESV7nsOwxleKZZjQk/GjS2x9Wviw+7YEw3Lrdq291TFvgPqXFiUv3baROxA39uCYTcft2LFvaA2l0pTwB", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [Distributor_Code = _t, Division = _t, #"Prim Item Code" = _t, Date = _t, #"Open Stock" = _t, #"Prim Sales" = _t, #"Sec Sales" = _t]),
    #"Changed Type" = Table.TransformColumnTypes(Source,{{"Distributor_Code", Int64.Type}, {"Division", type text}, {"Prim Item Code", Int64.Type}, {"Date", type date}, {"Open Stock", type number}, {"Prim Sales", Int64.Type}, {"Sec Sales", type number}}),
//add index column to be able to sort back to original order
    #"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),

//Group by "dist code", "division", and "prim item code"
// then add a column to each subgroup for the closing stock based on rules in your question.
    #"Grouped Rows" = Table.Group(#"Added Index", {"Distributor_Code", "Division", "Prim Item Code"}, {
        
        {"add cs", (t)=>
            Table.FromColumns(
                Table.ToColumns(t) & 
                {List.Generate(
                    ()=>[cs=t[Open Stock]{0} + t[Prim Sales]{0} - t[Sec Sales]{0}, idx = 0],
                    each [idx] < Table.RowCount(t),
                    each [cs= let 
                                Open = if t[Open Stock]{[idx]+1} = 0 then [cs] else t[Open Stock]{[idx]+1}
                             in 
                                Open + t[Prim Sales]{[idx]+1} - t[Sec Sales]{[idx]+1}, idx = [idx]+1],
                    each [cs])}, {"Distributor_Code", "Division", "Prim Item Code", "Date", 
            "Open Stock", "Prim Sales", "Sec Sales", "Index", "Closing Stock"})}          
            }),

//expand the grouped table
//sort back to original order
//remove the Index column
    #"Expanded add cs" = Table.ExpandTableColumn(#"Grouped Rows", "add cs", {"Date", "Open Stock", "Prim Sales", "Sec Sales", "Index", "Closing Stock"}),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded add cs",{{"Date", type date},{"Open Stock", type number}, {"Prim Sales", Int64.Type}, {"Sec Sales", type number}, {"Index", Int64.Type}, {"Closing Stock", type number}}),
    #"Sorted Rows" = Table.Sort(#"Changed Type1",{{"Index", Order.Ascending}}),
    #"Removed Columns" = Table.RemoveColumns(#"Sorted Rows",{"Index"})
in
    #"Removed Columns"