Fill Down on conditions

See file for example.

let
    Source = Excel.CurrentWorkbook(){[Name="pay_test"]}[Content],
    changeType = Table.TransformColumnTypes(Source,{{"Emp. #", type text}, {"Employment Type", type text}, {"Type", type text}, {"Visit ID", Int64.Type}, {"Employee Cost Centre", type any}, {"Cost Centre Description", type any}, {"Visit start date", type text}, {"Visit end date", type text}, {"Visit start time", type time}, {"Visit end time", type time}, {"Service type", type text}, {"Earning Code", type text}, {"Quantity", type number}, {"Premium Quantity", type any}, {"Units", type text}, {"Override rate", Int64.Type}, {"Pay amount", Int64.Type}}),
    priorStep = Table.SelectRows(changeType, each ([Service type] <> "Reimburse KMs Travelled")),
    eligile_index =
        let 
            buffer_date = List.Buffer(priorStep[Visit start date]), 
            buffer_start = List.Buffer(priorStep[Visit start time]),
            buffer_end = List.Buffer(priorStep[Visit end time])
        in
            List.Generate(
                () => {0}, // current, index
                each _{0} < List.Count(buffer_start) - 1,
                each { _{0} + 1},
                each 
                    let x = (buffer_start{_{0}+1} ?? buffer_start{_{0}+2}) - buffer_end{_{0}}
                    in if buffer_start{_{0}} <> null 
                    then Duration.TotalMinutes(x) < 60 and Duration.TotalMinutes(x) >= 0
                    else null
            ),
    add_eligible_index = 
        Table.Buffer(
            Table.FromColumns(
                Table.ToColumns(priorStep)&{eligile_index},
                Table.ColumnNames(priorStep)&{"Eligible"}
            )
        ),
/*    #"Added Index" = Table.AddIndexColumn(#"Filtered Rows", "Index", 0, 1, Int64.Type),
    #"Added Eligible" = let buffer = #"Added Index"[Visit start time] in Table.AddColumn(#"Added Index", "Eligible", each if [Visit end time] <> null 
then Duration.TotalMinutes(
    (buffer{[Index]+1}
    ?? buffer{[Index]+2}) - [Visit end time]) < 60 else null),
*/
    priorStep2 =Table.FillDown(add_eligible_index,{"Visit start date", "Visit end date"}),
    null_index =
        let
            buffer_emp = List.Buffer(priorStep2[#"Emp. #"]), // needed for employee id condition
            buffer_date = List.Buffer(priorStep2[Visit start date]),
            buffer_start = List.Buffer(priorStep2[Visit start time]),
            buffer_code = List.Buffer(priorStep2[Earning Code]),
            buffer_eligible = List.Buffer(priorStep2[Eligible]),
            buffer_end = List.Buffer(priorStep2[Visit end time])
        in
            List.Generate(
                () => {0,0,{},buffer_date{0}}, // current row, null index, non null list, current date
                each _{0} < List.Count(buffer_end),
                each  
                    {
                        _{0}+1, 
                        if 
                            buffer_date{_{0}+1} <> _{3} 
                            or buffer_emp{_{0}+1} <> buffer_emp{_{0}}
                        then List.Count(_{2})
                        else if buffer_code{_{0}} = "TTBV" then _{1} + 1 else _{1},
                        if 
                            try buffer_code{_{0}} <> "TTBV"
                            and (buffer_eligible{_{0}} = true and buffer_eligible{_{0}} <> null)
                            otherwise false 
                        then List.Buffer(_{2} & {_{0}}) 
                        else List.Buffer(_{2}),
                        buffer_date{_{0}+1}
                    }, // create list of all non null values.  create index to select from non null values.  increase index when null value is found or take last non null value when visit start date changes so you can pick the first non null value after the date change.
                each if buffer_code{_{0}} <> "TTBV" then null else try buffer_end{_{2}{_{1}}} otherwise null
            ),
    add_null_index = 
        Table.Buffer(
            Table.FromColumns(
                Table.ToColumns(priorStep2)&{null_index},
                Table.ColumnNames(priorStep2)&{"replace_start"}
            )
        ),
    #"Changed Type" = Table.TransformColumnTypes(add_null_index,{{"replace_start", type time}}),
    #"Replaced Value" = Table.ReplaceValue(#"Changed Type",each [Visit start time],each [replace_start] ?? [Visit start time],(origV,oldV,newV) as time => newV,{"Visit start time"}),
    #"Added Custom" = Table.AddColumn(#"Replaced Value", "end_replace", each [Visit end time] ?? 
Time.From(
    Number.Mod(
        Number.From([Visit start time]) 
        + Number.RoundDown([Quantity],0)/(24*60) 
        + Number.Mod([Quantity],1)*100/(24*60*60)
    ,1)
), type time),
    #"Replaced Value1" = Table.ReplaceValue(#"Added Custom",each [Visit end time],each [end_replace],(origV,oldV,newV) as time => newV,{"Visit end time"})
in
    #"Replaced Value1"

https://docs.google.com/spreadsheets/d/1Ym6APkXeS0gyx_PxSMmCSM8J8hVNQqDj/edit?usp=sharing&ouid=114317101938718300746&rtpof=true&sd=true