Fill down with index value

let
  Source = Table.FromRows(
    Json.Document(
      Binary.Decompress(
        Binary.FromText(
          "Vc5LDsAgCATQu7B2IShKz2K8gemu9++UJiTs3vAJrEVMhSrtskggdjVIXB1qLoW6a0DqmtBwGTSjZlG7YoNrtPm/cj/nfElSain1+II13uCRZmZKlhLu7xc=", 
          BinaryEncoding.Base64
        ), 
        Compression.Deflate
      )
    ), 
    let
      _t = ((type nullable text) meta [Serialized.Text = true])
    in
      type table [Custom = _t, Index.1 = _t]
  ), 
  #"Changed Type" = Table.TransformColumnTypes(
    Source, 
    {{"Index.1", Int64.Type}, {"Custom", Int64.Type}}
  ), 
  #"Replaced Value" = Table.ReplaceValue(
    #"Changed Type", 
    each [Custom], 
    (k) =>
      if k[Custom] = null then
        let
          t = Table.Last(
            Table.SelectRows(#"Changed Type", each [Index.1] < k[Index.1] and [Custom] <> null)
          )
        in
          k[Index.1] - t[Index.1] + t[Custom]
      else
        k[Custom], 
    Replacer.ReplaceValue, 
    {"Custom"}
  )
in
  #"Replaced Value"

How to use this code: Create a new Blank Query. Click on "Advanced Editor". Replace the code in the window with the code provided here. Click "Done". Once you examined the code, replace the Source step with your own source.