Ciao, è molto semplice...

Sub Centraform(FormName As Form)
On Error Resume Next
If FormName.MDIChild = True Then
FormName.top = (Menu00.ScaleHeight / 2 - FormName.Height / 2)
FormName.left = (Menu00.ScaleWidth / 2 - FormName.Width / 2)
If FormName.top < 0 Then FormName.top = 0
Else
FormName.top = (Menu00.Height / 2 - FormName.Height / 2) + Menu00.top
FormName.left = (Menu00.Width / 2 - FormName.Width / 2)
FormName.left = FormName.left + Menu00.left
If FormName.top < 0 Then FormName.top = 0
If FormName.left < 0 Then FormName.left = 0
End If
End Sub