Why not to add into B4XView class 2 methods of the Visual Designer ?
B4X:
Sub setHorizontalCenter(v As B4XView, HorizontalPercent As Float)
v.Left = v.Parent.Width / 100 * HorizontalPercent - (v.Width/2)
End Sub
Sub setVerticalCenter(v As B4XView, VerticalPercent As Float)
v.Top = v.Parent.Height / 100 * VerticalPercent - (v.Height/2)
End Sub