Create new column based on criteria and previous value

in DAX

=CALCULATE(COUNT(Table3[Time]),'Table3'[Time]<>0,ALLSELECTED(Table3[Description]))-CALCULATE(COUNT(Table3[Time]),'Table3'[Time]<>0,ALLSELECTED(Table3[Description]),'Table3'[Description]>=MAX('Table3'[Description]))+1

in M

let
    Source = Excel.CurrentWorkbook(){[Name="Table3"]}[Content],
    Custom1 = let a=List.Buffer(Source[ Time]) in List.Generate(()=>{1,0},each _{1}<Table.RowCount(Source),each {_{0}+Byte.From(a{_{1}}<>null),_{1}+1},each _{0}),
    Custom2 = Table.FromColumns(Table.ToColumns(Source)&{Custom1},Table.ColumnNames(Source)&{"CaseID"})
in
    Custom2