Autofill with variable (dynamic) range VBA

Sub Last_Real_Populated_Column()
 
 
 ActiveCell.SpecialCells(xlLastCell).Select
 LastR = ActiveCell.Row
 LastC = ActiveCell.Column
 LastRealC = 1
 For Counter = LastC To 1 Step -1
    Range(Cells(LastR, Counter), Cells(LastR, Counter)).Select
     Selection.End(xlUp).Select
    If Not IsEmpty(ActiveCell.Value) Then
        LastRealC = ActiveCell.Column
         Exit For
    End If
Next
Selection.End(xlUp).Offset(1, 0).Select
 
ActiveCell.Select
debut = ActiveCell.Address
fin = ActiveCell.Offset(200, 0).Address
Range(debut, fin).Select
 
ActiveCell.Select
Selection.AutoFill Destination:=Range(debut, fin), Type:=xlFillDefault
 
ActiveCell.Offset(0, -1).Select
debut = ActiveCell.Address
fin = ActiveCell.Offset(200, 0).Address
Range(debut, fin).Select
 
ActiveCell.Select
Selection.AutoFill Destination:=Range(debut, fin), Type:=xlFillDefault

Sub testone()
Dim lastc As String, j, firstc As String, sstep
'lastc is lastcell
 firstc = InputBox("type firsts cell e.g. A1")
j = InputBox("type data you want to enter in firstcell, d.g. 1 or 3 etc")
lastc = InputBox("type the upto lastcell you want to auttofil,e.g.A10")
sstep = InputBox("type the step e.g. 1 or 2 or 3 as you like")
Range(firstc) = j
Range(firstc, lastc).DataSeries Rowcol:=xlColumns, Type:=xlLinear, step:=sstep, Trend:=False
End Sub