Auto fill not working - vba

Question

I am working on my code and I have this to filter rows and insert a formula in the first filter row. with that formula I want it to fill down, but it only insert the formula in the first filtered row and doesn't fill down.
Sub Cal()
dim LastRow as long
With Worksheets("Data")
.Range("$A$1:$AI$80000").AutoFilter Field:=1, Criteria1:= _
"Actual"
.Range("$A$1:$AI$80000").AutoFilter Field:=2, Criteria1:="2018"
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
.AutoFilter.Range.Offset(2).SpecialCells(xlCellTypeVisible).Cells(1, 35).Select 'SELECTS THE FIRST cell in A after deleting
ActiveCell.FormulaR1C1 = "=SUM(RC[-12]:RC[-1])"
.AutoFilter.Range.Offset(2).SpecialCells(xlCellTypeVisible).Cells(1, 35).Select
Selection.FillDown
End With
End Sub

Answer 1

How about something like below, instead of .FillDown, specify the range for the last column of visible data, and offset to the next column to enter the formula in there:
Sub Cal()
Dim LastRow As Long
With Worksheets("Data")
LastRow = .Cells(.Rows.Count, "A").End(xlUp).Row
.Range("$A$1:$AI$" & LastRow).AutoFilter Field:=1, Criteria1:="Actual"
.Range("$A$1:$AI$" & LastRow).AutoFilter Field:=2, Criteria1:="2018"
'filter according to values specified
Set fltrdrng = .Range("$AI$2:$AI$" & LastRow).SpecialCells(xlCellTypeVisible)
'set the range of visible data on last column with data on your data-set
fltrdrng.Offset(0, 1).FormulaR1C1 = "=SUM(RC[-12]:RC[-1])"
'add the formula to the adjacent column by offsetting
End With
End Sub

Answer 2

This should do what you want. Autofilling is very dangerous with filtered data. This gets the activecell address and then creates the last cell address(row 80000 from your code), and then adds the formula to every cell in the range that is visible.
Start = ActiveCell.address
arow = ActiveCell.Row
alen = Len(arow)
lcell = Left(Start, Len(Start) - alen) & "80000"
Range(Start & ":" & lcell).SpecialCells(xlCellTypeVisible).Formula = "=SUM(RC[-12]:RC[-1])"