How to add a custom column with surrogate grouping

Hi kirvis - Take a look at this code:

let
    Source = {1..100},
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), null, null, ExtraValues.Error),
    #"Added Index" = Table.AddIndexColumn(#"Converted to Table", "Index", 1, 1/3, Int64.Type),
    #"Inserted Round Down" = Table.AddColumn(#"Added Index", "Round Down", each Number.RoundDown(Number.Round([Index],8)), Int64.Type)
in
    #"Inserted Round Down"

What I did is:

• Create a fake column of 1 to 100 just to have some data.
• Added an index. Then I changed the index to start at 1 and increment by 1/3.
• Then I added a column with this formula: Number.RoundDown(Number.Round([Index],8))

It returns this:

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.