Parent-child flattening (multiple levels)

bartvana 

I didn't dive a deep into calculations, here is only suggestion for iterations.

We keep as it is now tBOM and tCalc1.

Levels are iterated by fntCalcQty:

( tCalc as table, tBasis as table, tAccumulate as table ) =>

let
    nextLevel = fnCalc( tCalc, tBasis ),
    addTable = Table.Combine( {tAccumulate, nextLevel} )

in
    if  Table.RowCount( nextLevel ) = 0
    then tAccumulate
    else fntCalcQty (nextLevel, tBasis, addTable)

It iterates till first empty table appears.

Final result is calculated as

let
    Source = fntCalcQty( tCalc1, tBOM, tCalc1),
    addSales = Table.Combine( {Source, tSalesRenamed} ), 
    #"Grouped Rows" = Table.Group(
        addSales, {"Subpart"},
        {{"TotalQty", each List.Sum([TotalQty]), type nullable number}}
    ),
    #"Sorted Rows" = Table.Sort(#"Grouped Rows",{{"Subpart", Order.Ascending}})
in
    #"Sorted Rows"

Above is in Variant 03 group in attached.