Thanks for sharing the details, Henk! You're very close, the main issue is where and how you declare your public variable.
You must put Public declarations in a standard module, not inside the ThisWorkbook object or a worksheet module, otherwise it will be private to that object and inaccessible elsewhere.
So insert a module, then enter this:
' Module1
Option Explicit
Public sMuis As Integer
Then, in your ThisWorkbook code, you can set or use that variable when the workbook opens:
' ThisWorkbook
Option Explicit
Private Sub Workbook_Open()
sMuis = 10 ' or any value you want
Sheets("Home").Select
End Sub
See if this helps. If you need further assistance just let me know.
Best regards,
Kimberly