Excel Stops Recalculating

The Tip says, "...if your Personal workbook...is saved with the calculation mode set to manual, then every time you start Excel directly...your entire session in Excel will be in manual recalculation mode." I believe this applies only if you create a new workbook (e.g., click New in Backstage) before opening an existing workbook. Otherwise, the existing workbook's calculation mode will apply. This can be tested with the Personal.xlsb code in my earliest comment below. Of course, you can always change the calculation mode to automatic by picking Formulas > Calculation Options > Automatic (Alt+M+X+A) at any time during your session.
Re. my code for Personal.xlsb, the following statement
    If Wb.Name = ThisWorkbook.Name Then Exit Sub
can be replaced by this statement
    If Wb Is ThisWorkbook Then Exit Sub
for a minor improvement.
The following formula uses Excel's INFO function to report the current calculation mode:
    =INFO("RECALC")
And this function in My Excel Toolbox adds a comment (Note) to the formula's cell indicating the current calculation mode:
    =CalcMode([Visible])
If optional Visible is FALSE, the comment will not be made visible; the default is TRUE, which makes it visible. CalcMode returns Null (""/0/FALSE), so its value can be ignored when combined with another function. For example,
    =PI() + CalcMode()    --    returns 3.14159... with a comment
    ="abc" & CalcMode()    --    returns abc with a comment
After using Formulas > Calculation Options > Manual (Alt+M+X+M) to switch from Automatic, it is necessary to press F9 to update INFO or CalcMode.
Finally, My Excel Toolbox includes the ToggleCalcMode macro (Ctrl+T+C+M) to switch from Automatic (or Semiautomatic) to Manual and from Manual to Automatic; in either case it recalculates all open workbooks (updating any INFO or CalcMode formulas). And it briefly displays a calculation mode notification (BalloonTip) in the system tray and action center.
See https://sites.google.com/view/MyExcelToolbox/

@Simon Freeman
Sub #1: When Personal.xlsb opens (it is usually first), give App the ability to process Application events.
Sub #2: When a new workbook is created (e.g., click New in Backstage), use Sub #3 to check if Calculation is Manual.

J. Woolley - could you kindly explain the first two subs ie

Private Sub Workbook_Open()
Set App = Application
End Sub

Private Sub App_NewWorkbook(ByVal Wb As Workbook)
App_WorkbookOpen Wb
End Sub

Many thanks

Simon

J. Woolley - many thanks - Simon

@Simon Freeman
Yes. Press Alt+F11 to open the Visual Basic Editor (VBE), then copy/paste the following code to the ThisWorkbook module of PERSONAL.XLSB:

Private WithEvents App As Application

Private Sub Workbook_Open()
    Set App = Application
End Sub

Private Sub App_NewWorkbook(ByVal Wb As Workbook)
    App_WorkbookOpen Wb
End Sub

Private Sub App_WorkbookOpen(ByVal Wb As Workbook)
    Dim msg As String
    If Wb.Name = ThisWorkbook.Name Then Exit Sub
    If Application.Calculation = xlCalculationManual Then
        msg = "The calculation mode is Manual." & vbLf _
            & "Do you want to make it Automatic?"
        If MsgBox(msg, (vbQuestion + vbYesNo)) = vbYes Then
            Application.Calculation = xlCalculationAutomatic
            msg = "The calculation mode is now Automatic."
            MsgBox msg, vbInformation
        End If
    End If
End Sub

For more about PERSONAL.XLSB, see:
https://www.myonlinetraininghub.com/create-a-personal-macro-workbook-personal-xlsb
https://www.ablebits.com/office-addins-blog/excel-personal-macro-workbook/

Would it be possible to create a macro that runs whenever you open a workbook and if the workbook is set to manual calculation, warns you and invites you to change it to auto. It shouldn't change it automatically as you might want manual calculation.