Having problems passing parameters into functions

I have this function - tested and working

let
    // accepts two parameters
    // PositionInFY - Position in Financial year month parade of the variable we're trying to find - This FY October = 1, This FY November = 2 and so on up to 24 for each month in the 2 years
    // PositionInYear - Position of month in calendar year with a leading zero if necessary
    // eg GetFirstDayOfMonth(19, 04) for next april
    GetFirstDayOfMonth = (_PositionInFY as number, _PositionInYear as number) =>
    let
        _Today = DateTime.Date(DateTime.LocalNow()),  // Obtain today's date
        _MonthOfToday = Date.Month(_Today),           // Obtain the month of today
        _YearOfToday = Date.Year(_Today),             // Obtain year of today
        // _PositionInYear = Text.From (_PositionInYear),
        _YearInFY = 
            if _MonthOfToday >= 10 then
                if _PositionInFY <= 3 then _YearOfToday
                else if _PositionInFY > 3 and _PositionInFY < 16 then _YearOfToday + 1
                else if _PositionInFY >= 16 then _YearOfToday + 2
                else null
            else
                if _PositionInFY <= 3 then _YearOfToday - 1
                else if _PositionInFY > 3 and _PositionInFY < 16 then _YearOfToday
                else if _PositionInFY >= 16 then _YearOfToday + 1
                else null,

        _GetFirstDayOfMonth = Date.FromText(Number.ToText(_YearInFY) & "-" & Number.ToText(_PositionInYear) & "-01")  // First day of Month
    in
        _GetFirstDayOfMonth
in
    GetFirstDayOfMonth

Then I want to create a second fundtion called ValueThisMonth that calls the GetFirstDayOfMonth function.

To start slowly, I started with this - putting all the parameters in and it works.  I expect to get 10,000 as the answer and I do.

let
        _PositionInFY = 2,
        _PositionInYear = 11,        
        FirstDayThisMonth = (GetFirstDayOfMonth(_PositionInFY, _PositionInYear)),
        LastDayThisMonth = Date.EndOfMonth(FirstDayThisMonth),
        DaysThisMonth = Duration.Days(LastDayThisMonth - FirstDayThisMonth),
        StartDate = Date.From("2023-11-01"),
        EndDate = Date.From("2023-11-30"),
        Duration = 30,
        Value = 10000,
        ValueThisMonth =
                
                        if StartDate < FirstDayThisMonth and EndDate < FirstDayThisMonth then 0
                        else if StartDate < FirstDayThisMonth and EndDate >= FirstDayThisMonth and EndDate <= LastDayThisMonth then
                            Number.Round((((Duration.Days(EndDate-FirstDayThisMonth)+1) / Duration) * Value), 0)
                        else if StartDate >= FirstDayThisMonth and EndDate <= LastDayThisMonth then
                            Number.Round((((Duration.Days(EndDate-StartDate)+1) / Duration) * Value), 0)
                        else if StartDate >= FirstDayThisMonth and StartDate <= LastDayThisMonth and EndDate > LastDayThisMonth then
                            Number.Round((((Duration.Days(LastDayThisMonth-StartDate)+1) / Duration) * Value), 0)
                        else if StartDate < FirstDayThisMonth and EndDate > LastDayThisMonth then Number.Round((DaysThisMonth / Duration) * Value, 0)
                        else if StartDate > LastDayThisMonth then 0
                        else 0
    in
        ValueThisMonth

But when I attempt to pass in the parameters it doesn't work...

let
    ValueThisMonth = (_PositionInFY as number, _PositionInYear as number) =>
    let
        FirstDayThisMonth = GetFirstDayOfMonth(_PositionInFY, _PositionInYear),
        LastDayThisMonth = Date.EndOfMonth(FirstDayThisMonth),
        DaysThisMonth = Duration.Days(LastDayThisMonth - FirstDayThisMonth),
        StartDate = Date.From(2023-11-01),
        EndDate = Date.From(2023-11-31),
        Duration = 30,
        Value = 10000,
        ValueThisMonth =
            if StartDate < FirstDayThisMonth and EndDate < FirstDayThisMonth then 0
            else if StartDate < FirstDayThisMonth and EndDate >= FirstDayThisMonth and EndDate <= LastDayThisMonth then
                Number.Round((((Duration.Days(EndDate-FirstDayThisMonth)+1) / Duration) * Value), 0)
            else if StartDate >= FirstDayThisMonth and EndDate <= LastDayThisMonth then
                Number.Round((((Duration.Days(EndDate-StartDate)+1) / Duration) * Value), 0)
            else if StartDate >= FirstDayThisMonth and StartDate <= LastDayThisMonth and EndDate > LastDayThisMonth then
                Number.Round((((Duration.Days(LastDayThisMonth-StartDate)+1) / Duration) * Value), 0)
            else if StartDate < FirstDayThisMonth and EndDate > LastDayThisMonth then Number.Round((DaysThisMonth / Duration) * Value, 0)
            else if StartDate > LastDayThisMonth then 0
            else 0
    in
        ValueThisMonth
in
    ValueThisMonth

Any help getting that to work would be greatly appreciated.

Then the next step will be to pass in parameters for the other variables which are held on a table as follows

StartDate = 'MSD Opportunity'[start_date]
EndDate = 'MSD Opportunity'[end_date]
Duration = 'MSD Opportunity'[Duration]
Value = 'MSD Opportunity'[value]

I have tried Chat GPT to help me but no joy.  Any help from humans greatly appreciated.