Reverse cumulative by group

You can first perform the calculations without grouping, then turn that query into a function,

Then use that function in combination with Table.Group.

This 8 minute video illustrates the steps in a similar case, including some specifics I also used in this case (buffering, using Value.Type).

By the way, there is some discrepancy between your text and your formula, as Counts are summed with an Index greater than the current Index, not with a Score greater than the current Score.

Function Calculate:

(Table as table) as table =>
let
    Source = Table,
    Counts = List.Buffer(Source[Count]),
    AddedCalc = Table.AddColumn(Source, "Calc", each List.Sum(List.Skip(Counts,[Index])&{0}) / List.Sum(Counts), Percentage.Type)
in
    AddedCalc

Query Output (Table1 is your data, already available as a query in Power Query):

let
    Source = Table.Buffer(Table1),
    #"Grouped Rows" = Table.Group(Source, {"Month"}, {{"AlRows", Calculate, Value.Type(Calculate(Source))}}),
    #"Removed Columns" = Table.RemoveColumns(#"Grouped Rows",{"Month"}),
    #"Expanded AlRows" = Table.ExpandTableColumn(#"Removed Columns", "AlRows", {"Month", "Score", "Count", "Index", "Calc"})
in
    #"Expanded AlRows"