Calculate only selected row - excel

Question

I have a large workbook and am trying to increase performance.
Is it possible/viable to store my formulas in some sort of list contained within the code rather than in the cells on the spreadsheet?
Variable SelectedRow = the currently selected row
For example:
ColumnBFormula = A(SelectedRow) + 1
ColumnCFormula = A(SelectedRow) + 2
If the user enters 4 in cell A3, then the macro writes formulas above ONLY in empty cells B3 and C3, then converts to values. The rest of the spreadsheet remains unchanged (should only have values everywhere).
Then the user enters a 6 in cell A4 and the spreadsheet writes the formulas to empty cells B4 and C4, calculates then converts to values.
Thanks

Answer 1

Try:
Private Sub Worksheet_Change(ByVal Target As Range)
Dim Lastrow As Long
'Refer to Sheet1
With ThisWorkbook.Worksheets("Sheet1")
'Check if Column A affected
If Not Intersect(Target, Range("A:A")) Is Nothing And IsNumeric(Target) Then
'Disable event to avoid event trigger
Application.EnableEvents = False
Target.Offset(0, 1).Value = Target + 1
Target.Offset(0, 2).Value = Target + 2
'Enable event
Application.EnableEvents = True
End If
End With
End Sub
Instructions:
Enable Events:

Answer 2

Given you know what you want the code to do, you could do this without entering formulas.
In the VBA editor, add this code into the "ThisWorkbook" object ...
Private Sub Workbook_SheetChange(ByVal Sh As Object, ByVal Target As Range)
Dim objCell As Range
Application.EnableEvents = False
For Each objCell In Target.Cells
If objCell.Column = 1 Then
If objCell.Value = "" Then
objCell.Offset(0, 1) = ""
objCell.Offset(0, 2) = ""
Else
objCell.Offset(0, 1) = objCell.Value + 1
objCell.Offset(0, 2) = objCell.Value + 2
End If
End If
Next
Application.EnableEvents = True
End Sub
Hopefully that works for you.
FYI - You'll need to add the relevant error checking for values if not numeric etc, it will need to be improved.