Search Code To Find The Last Formula Used

Code such as the one below carries formulas in columns 3 and 10 down to new rows when inserted. However, I need to allow a user to overwrite the formula in col 3 with any text value. But if the user overwrites the formula on the last row, and then manually enters a new record below, then the code in the last row of col 3 does not get copied down. Is there a way to search upward in columns 3 and 10 until reaching the last cells with formulas, and apply the formulas to the new cells in cols 3 and 10 on the new row? Or, is there a way to store the formulas:

=IF(A21<>"",B21,"") in column 3
=IF(B21<>0,IF(C21<>B21,A21&", "&LEFT(C21,2),A21&", "&LEFT(B21,1)),"") in column 10

Storing these somehow would be good if the user overwrites the formulas in the very top row of data (row 2).

If Intersect(Target, Me.Range("A:A")) Is Nothing Then Exit Sub
On Error Goto CleanUp:
With Target
If .Value <> "" Then
Application.EnableEvents = False
.Offset(0, 3).FormulaR1C1 = "=RC[-1]=R2C3"
.Offset(0, 10).FormulaR1C1 = "=RC[-1]=R2C10"
End If
End With
CleanUp:
Application.EnableEvents = True