Replace null values with conditional average

let
    UnitsSold = Excel.CurrentWorkbook(){[Name="Table1"]}[Content],
    EmployeeDetails = Table.Buffer(Excel.CurrentWorkbook(){[Name="Table2"]}[Content]),
    Custom1 = #table(
                     Table.ColumnNames(UnitsSold),
                     List.Accumulate(
                                     Table.ToRows(UnitsSold),
                                     {{},{},""},
                                     (x,y)=>let
                                               a=EmployeeDetails{[Employee=y{0},Employment Status="Resigned"]}?[#"Emp. End date"]? ??DateTime.LocalNow(),
                                               b=List.Average(List.LastN(x{1},3))
                                            in
                                               if y{1}>=a then {x{0}&{y},{},y{0}} else
                                               if y{0}<>x{2} then {x{0}&{y},{y{2}},y{0}} else
                                               if y{2}<> null then {x{0}&{y},x{1}&{y{2}},y{0}}
                                               else {x{0}&{List.FirstN(y,2)&{b}},x{1}&{b},y{0}}
                                    ){0}
                    )
in
    Custom1