Collecting variable data from cells

Ah.  You wrote to calculate length.  But you really want length and width.

Try this (explanation later if needed)

let
    Source = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    #"Changed Type" = Table.TransformColumnTypes(Source,{
        {"INDEX3D", Int64.Type}, {"length", Int64.Type}, {"edge1", type text}, {"edge2", type text}, {"width", Int64.Type}, {"edge3", type text}, {"edge4", type text}, {"Type", type text}, {"Q.", Int64.Type}}),
    #"Length x Quantity" = Table.AddColumn(#"Changed Type", "Length x Quantity", each [length] * [#"Q."], Int64.Type),
    #"Width x Quantity" = Table.AddColumn(#"Length x Quantity","Width x Quantity", each [width] * [#"Q."], Int64.Type),
    #"Filtered Rows" = Table.SelectRows(#"Width x Quantity", each ([Type] = "edge")),
    #"Removed Columns" = Table.RemoveColumns(#"Filtered Rows",{"length", "width", "Q.", "INDEX3D", "Type"}),
    #"Unpivoted Other Columns" = Table.UnpivotOtherColumns(#"Removed Columns", {"Length x Quantity", "Width x Quantity"}, "Attribute", "Value"),
    #"Added Custom" = Table.AddColumn(#"Unpivoted Other Columns", "Lengths", each if [Attribute]="edge1" or [Attribute]="edge2" then [Length x Quantity] else [Width x Quantity]),
    #"Removed Columns1" = Table.RemoveColumns(#"Added Custom",{"Length x Quantity", "Width x Quantity", "Attribute"}),
    #"Grouped Rows" = Table.Group(#"Removed Columns1", {"Value"}, {{"Total Length", each List.Sum([Lengths]), type number}})
in
    #"Grouped Rows"