Trying to add more rows to a table based on existing data

I would like to be able to use this code to add more rows with these formulas to the bottom of the already existing data. I have been able to get things relative to the first cell, but I dont know how to make the first cell relative to the end of the previously entered data.

Sub test()

Sheets("Logbook").Select

Range("n4").Select
ActiveCell.FormulaR1C1 = _
"=IF(ISBLANK(rc[-12]),"""",VLOOKUP(rc[-12],Tail_Make,2))"

' Cross Country look up
ActiveCell.Offset(columnOffset:=5).Activate
ActiveCell.FormulaR1C1 = _
"=IF(ISNUMBER(RC[179]),IF(RC[179]>50,""Y"",""""),"""")"

ActiveCell.Offset(columnOffset:=3).Activate
ActiveCell.FormulaR1C1 = _
"=IF(ISBLANK(RC[-15]),"""",ROUND(IF(RC[-15]>RC[-14],(23-HOUR(RC[-15]-RC[-14]))+(60-MINUTE(RC[-15]-RC[-14]))/60,HOUR(RC[-14]-RC[-15])+MINUTE(RC[-14]-RC[-15])/60),2))"

Range("A4:v4").Select
Selection.AutoFill Destination:=Range("A4:v51"), Type:=xlFillDefault
End Sub