Problem with add a sheet to workbook macro

I have a workbook where I have a button to add a sheet, this works fine except for a formula that the macro edits on every sheet so we can have a running total quantity for the project. The problems occurs if the user changes the name of the last sheet in the workbook, then adds a sheet then the formula gets a reference error. The sheets are named 1 , 2 ,3 and so on, but it is nice to be able to change tha name of the sheet to the date.

Here is the code that Im have problems with

 ' Dim ShtName As String
    ShtName = Sheets.Count - 3
    Range("F36:F37").Select
    ActiveCell.FormulaR1C1 = "=R[-2]C+'" & ShtName & "'!RC:R[1]C"

is there a code that would always reference the the previous sheet or I know that every sheet has 2 names a permanent sheet name that is Sheet1 Sheet2 and so on and the name that is shown on the tab at the bottom of the workbook, is there a way I can reference the permanent sheet name that would be a variable because I always need this formula to go to the previous page.

Any help is appreciated.
Thanks