Power Query
let
Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
#"Changed Type" = Table.TransformColumnTypes(Source,{{"Level Number", Int64.Type}, {"Part #", type text}}),
#"Added Custom" = Table.AddColumn(#"Changed Type", "Custom", each [Level Number]-1),
#"Filtered Rows" = Table.SelectRows(#"Added Custom", each ([Custom] = -1 or [Custom] = 0 or [Custom] = 1)),
#"Added Conditional Column" = Table.AddColumn(#"Filtered Rows", "Custom.1", each if [Custom] = -1 then "--" else [#"Part #"] ),
#"Removed Columns" = Table.RemoveColumns(#"Added Conditional Column",{"Level Number", "Part #", "Custom"}),
#"Renamed Columns" = Table.RenameColumns(#"Removed Columns",{{"Custom.1", "Part # of Level Number minus 1"}})
in
#"Renamed Columns"
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