It should be something like this:
let
Source = Excel.Workbook(File.Contents("D:\Projects\Internal CRM\Leave Data.xlsx"), null, true),
Sheet1_Sheet = Source{[Item="Sheet1",Kind="Sheet"]}[Data],
#"Promoted Headers" = Table.PromoteHeaders(Sheet1_Sheet, [PromoteAllScalars=true]),
#"Changed Type" = Table.TransformColumnTypes(#"Promoted Headers",{{"Date", type date}, {"Type", type text}, {"Debit/Credit", type number}}),
RunningSum = List.Skip(List.Accumulate(#"Changed Type"[#"Debit/Credit"],{0},(sum,value) => sum & {List.Min({12,List.Last(sum)+value})})),
TableWithRunningSum = Table.FromColumns(Table.ToColumns(#"Changed Type")&{RunningSum},Value.Type(Table.AddColumn(#"Changed Type","Running Sum", each 0, type number)))
in
TableWithRunningSum
Specializing in Power Query Formula Language (M)
