Date.Week of year

You could subtract the minimum FW_Index for the year from the current FW_Index.

let
    // Enter the date in Advanced Editor for the first day of the earliest Fiscal Year in #date(yyyy,m,d) format
    StartDate = #date(2017, 12, 31),
    // Enter the desired range of years as List in Advanced Editor
    YearRange = {2018..2021},
    // In Advanced Editor, use one of the two options for this step (code out the one not used with //) to either hard code or dynamically calculate the years with 53 weeks.  Substitute a different evaluation criterion as needed.
    // YearsWith53Weeks = {2019},
    YearsWith53Weeks = List.Select(YearRange, each Date.DayOfWeekName(#date(_,11,30)) = "Saturday"),
    StartingTable = Table.FromColumns({YearRange}, {"Year"}),
    AddNumberOfWeeksColumn = Table.AddColumn(StartingTable, "Weeks", each if List.Contains(YearsWith53Weeks, [Year]) then 53 else 52),
    #"Changed Type4" = Table.TransformColumnTypes(AddNumberOfWeeksColumn,{{"Year", Int64.Type}, {"Weeks", Int64.Type}}),
    // In the Advanced Editor, enter the two patterns for 52 and 53 week years as a list of weeks per fiscal month
    AddListOfMonthAndWeekCounts = Table.AddColumn(#"Changed Type4", "Custom", each if [Weeks] = 53 then List.Zip({{1..12}, {4,4,5,4,4,5,4,4,5,4,5,5}}) else List.Zip({{1..12}, {4,4,5,4,4,5,4,4,5,4,4,5}})),
    #"Expanded Custom" = Table.ExpandListColumn(AddListOfMonthAndWeekCounts, "Custom"),
    #"Extracted Values" = Table.TransformColumns(#"Expanded Custom", {"Custom", each Text.Combine(List.Transform(_, Text.From), ";"), type text}),
    #"Split Column by Delimiter" = Table.SplitColumn(#"Extracted Values", "Custom", Splitter.SplitTextByDelimiter(";", QuoteStyle.Csv), {"FM", "NumWeeks"}),
    #"Changed Type" = Table.TransformColumnTypes(#"Split Column by Delimiter",{{"FM", Int64.Type}, {"NumWeeks", Int64.Type}}),
    AddListOfWeeksColumn = Table.AddColumn(#"Changed Type", "WeekInFM", each {1..[NumWeeks]}),
    #"Expanded Custom1" = Table.ExpandListColumn(AddListOfWeeksColumn, "WeekInFM"),
    #"Changed Type1" = Table.TransformColumnTypes(#"Expanded Custom1",{{"WeekInFM", Int64.Type}}),
    AddWeekIndex = Table.AddIndexColumn(#"Changed Type1", "FW_Index", 1, 1, Int64.Type),
    Add7DayListPerWeek = Table.AddColumn(AddWeekIndex, "WeekDay", each {1..7}),
    #"Expanded WeekDay" = Table.ExpandListColumn(Add7DayListPerWeek, "WeekDay"),
    #"Changed Type2" = Table.TransformColumnTypes(#"Expanded WeekDay",{{"WeekDay", Int64.Type}}),
    RemoveUnneededColumns = Table.RemoveColumns(#"Changed Type2",{"Weeks", "NumWeeks", "WeekDay"}),
    AddDayIndex = Table.AddIndexColumn(RemoveUnneededColumns, "DayIndex", 0, 1, Int64.Type),
    AddDatesBasedOnStartDateAndDayIndex = Table.AddColumn(AddDayIndex, "Date", each Date.AddDays(StartDate, [DayIndex]), type date),
    // New code starts here
    #"Grouped Rows" = Table.Group(AddDatesBasedOnStartDateAndDayIndex, {"Year"}, {{"MinFW", each List.Min([FW_Index]), Int64.Type}}),
    #"Merged Queries" = Table.NestedJoin(AddDatesBasedOnStartDateAndDayIndex, {"Year"}, #"Grouped Rows", {"Year"}, "Grouped Rows", JoinKind.LeftOuter),
    #"Expanded Grouped Rows" = Table.ExpandTableColumn(#"Merged Queries", "Grouped Rows", {"MinFW"}, {"MinFW"}),
    #"Added Custom" = Table.AddColumn(#"Expanded Grouped Rows", "WeekInYear", each [FW_Index] - [MinFW] + 1, Int64.Type)
in
    #"Added Custom"