I think that your problem comes from the fact you pass the form's name to the sub instead of a reference to the object. Simply write :

Sub FormCentr(Frm as Form) 'Frm is an object variable, passed by refernce
 With Frm
   .Top = X
   .Left = Y
 End With
End Sub

And call FormCentr Me 'Or any other form instance

Hope it helps