Así me salió
Public Sub CENTRAR_VENTANA(ucVentanita As Object, Formu As Form)
Dim isq As Long
isq = (Formu.Width - ucVentanita.Width) / 2
If isq < 0 Then isq = 0
arr = (Formu.Height - ucVentanita.Width) / 2
If arr < 0 Then arr = 0
ucVentanita.Left = isq - 150
ucVentanita.Top = arr - 100
End Sub