two column subtraction, first value is not correct

Hi All,

pls help me to get subtrction of two column.

i have these two column..

after this i added two index clumn and merge of these two index column 

my subtraction is coming correctly but first value it wrong it should be 0.53%..

first value should not be null, this must take first value of source column

pls help me to solve this issue, i am new here so need simple solution..

let
Source = #"15_EARLY_PW_BASELINE",
#"Removed Columns" = Table.RemoveColumns(Source,{"Discipline"}),
#"Grouped Rows" = Table.Group(#"Removed Columns", {"EARLY_PW_WK"}, {{"PW_EARLY_CUM", each List.Sum([PW_EARLY_BASELINE]), type nullable number}}),
#"Changed Type" = Table.TransformColumnTypes(#"Grouped Rows",{{"PW_EARLY_CUM", Percentage.Type}}),
#"Added Index" = Table.AddIndexColumn(#"Changed Type", "Index", 0, 1, Int64.Type),
#"Added Index1" = Table.AddIndexColumn(#"Added Index", "Index.1", 1, 1, Int64.Type),
#"Merged Queries" = Table.NestedJoin(#"Added Index1", {"Index"}, #"Added Index1", {"Index.1"}, "Added Index1", JoinKind.LeftOuter),
#"Expanded Added Index1" = Table.ExpandTableColumn(#"Merged Queries", "Added Index1", {"EARLY_PW_WK", "PW_EARLY_CUM", "Index", "Index.1"}, {"EARLY_PW_WK.1", "PW_EARLY_CUM.1", "Index.2", "Index.1.1"}),
#"Sorted Rows" = Table.Sort(#"Expanded Added Index1",{{"Index", Order.Ascending}}),
#"Inserted Subtraction" = Table.AddColumn(#"Sorted Rows", "Subtraction", each [PW_EARLY_CUM] - [PW_EARLY_CUM.1], Percentage.Type)
in
#"Inserted Subtraction"

thanks

rakesh