Power Query - Looping calculation

you are absolutely rigth!

let
    Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("i45WMjIwtFTSUTI0QCJidUDiRiCeJUQIyoRKGAJ5YFlDMGkElzAC8oDIHCQKFzSGCJqCBI3hoiYQURNUK02R1IJEYwE=", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [date = _t, device = _t, plan = _t, res = _t]),
    q = Table.TransformColumnTypes(Source,{{"date", Int64.Type}, {"device", Int64.Type}, {"plan", Int64.Type}, {"res", Int64.Type}}),
    calcres=List.Accumulate({0..Table.RowCount(q)-1},{}, (s,c)=> s& {List.Max({0,q[plan]{c} - (List.Sum(List.LastN(s,2))??0)})})
    ttc=Table.ToColumns(q),
    nm=Table.ColumnNames(q),
    tfc=Table.FromColumns(ttc&{calcres}, nm&{"calcres"})
in
   tfc

I resolved my above issue by creating a function, and then invoking the function following a group by step. I have pasted both codes below as reference to the community - but I would still be interested in a solution that hacks the M code - without having to create a function - if possible, to deepen my M knowledge.

Here is the code that works:

The function (named fnListAccumulate) - which is essentially the code provided by @Rocco_sprmnt21 in the accepted solution - with minor changes to convert to a function (red) :

(GrpTbl as table)=>

let
calcres=List.Accumulate({0..Table.RowCount(GrpTbl)-1},{}, (s,c)=> s& {List.Max({0,GrpTbl[plan]{c} - (List.Sum(List.LastN(s,2))??0)})}),
ttc=Table.ToColumns(GrpTbl),
nm=Table.ColumnNames(GrpTbl),
tfc=Table.FromColumns(ttc&{calcres}, nm&{"calcres"})
in
tfc

Here is the original table invoking the above function after a group the desired fields (Scenario/Product):

let

Source = Table.FromRows(Json.Document(Binary.Decompress(Binary.FromText("jZI7DsMgDIbvwpwBTJyEW1TqGGVq90q9/1AewTJgo0rEYfhC/gfnacC6YBbzfLk4H9/PO72dZeNaEgV2oEIBbNnemBuw/KXLEwiDAYtrTwwhXkIwIZ6YVWLWVjiq5xDTRZDEIT2if6j/WbHsRPtQY0RsJMFAxbW1sr3E5IRQsQ/MvuIemPvOPPzVP6cm/XNs0j/H6KwugY45bImqSaBjdukCdMw2iWDSP0f0/jml98+pqhoU97w4UNzfTGjvCErMURO6fg==", BinaryEncoding.Base64), Compression.Deflate)), let _t = ((type nullable text) meta [Serialized.Text = true]) in type table [date = _t, scenario = _t, product = _t, device = _t, plan = _t, res = _t]),

q = Table.TransformColumnTypes(Source,{{"date", Int64.Type}, {"device", Int64.Type}, {"plan", Int64.Type}, {"res", Int64.Type}, {"scenario", type text}, {"product", type text}}),
#"Grouped Rows" = Table.Group(q, {"scenario", "product"}, {{"Allrows", each _, type table [date=nullable number, scenario=nullable text, product=nullable text, device=nullable number, plan=nullable number, res=nullable number]}}),
#"Invoked Custom Function" = Table.AddColumn(#"Grouped Rows", "ListAccumulate", each FnListAccumulate([Allrows])),
#"Removed Other Columns" = Table.SelectColumns(#"Invoked Custom Function",{"ListAccumulate"}),
#"Expanded ListAccumulate" = Table.ExpandTableColumn(#"Removed Other Columns", "ListAccumulate", {"date", "scenario", "product", "device", "plan", "res", "calcres"}, {"date", "scenario", "product", "device", "plan", "res", "calcres"})
in
#"Expanded ListAccumulate"