M Code: Day of Fiscal Year (date table)

You can use this logic and get this result:

Here is sample code to paste in and test with:

let
    Source ={Number.From(#date(2020,5,1))..Number.From(#date(2027,4,30))},
    #"Converted to Table" = Table.FromList(Source, Splitter.SplitByNothing(), {"Date"}, null, ExtraValues.Error),
    #"Changed Type" = Table.TransformColumnTypes(#"Converted to Table",{{"Date", type date}}),
    #"Added Fiscal Year" = Table.AddColumn(#"Changed Type", "Fiscal Year", each if Date.Month([Date]) > 4 then Date.Year([Date]) + 1 else Date.Year([Date]), Int64.Type),
    #"Grouped Rows" = 
        Table.Group(
            #"Added Fiscal Year", 
            {"Fiscal Year"}, 
            {
                {
                    "All Rows",
                     each 
                        Table.AddIndexColumn(
                            Table.Sort(_, {"Date", Order.Ascending}),
                            "Day of Year",
                            1
                        ),
                        type table [Date=nullable date, Fiscal Year=number, Day of Year=number]
                }
            }
        ),
    #"Expanded All Rows" = Table.ExpandTableColumn(#"Grouped Rows", "All Rows", {"Date", "Day of Year"}, {"Date", "Day of Year"})
in
    #"Expanded All Rows"

Here is what I did:

First, I Grouped the data by a fiscal year column, so May 1, 2022 through April 30, 2023 would be in the Fiscal 2023 group. The aggregation was "All Rows"

Then I edited the M code. That Grouping will just have an "each _, type table..."
I wrapped that with Table.AddIndexColumn and Table.Sort (to ensure the dates were ascending)

Then I expanded everything but the Fiscal Year column. You will want to reorder your columns to put Fiscal Year where it belongs vs in the first column.

How to use M code provided in a blank query:
1) In Power Query, select New Source, then Blank Query
2) On the Home ribbon, select "Advanced Editor" button
3) Remove everything you see, then paste the M code I've given you in that box.
4) Press Done
5) See this article if you need help using this M code in your model.