AutoCAD .NET :: How To Jig Rotate A Block With Different Fixed Point

here is my jig rotate code

Protected Overrides Function Sampler(ByVal prompts As JigPrompts) As SamplerStatus Dim myPPR As PromptPointResult myPPR = prompts.AcquirePoint(myOpts) Dim curPos As Point3d curPos = myPPR.Value If curPos.IsEqualTo(BasePt) Then Return SamplerStatus.NoChange Else myMatrix = Geometry.Matrix3d.Displacement( _ BasePt.GetVectorTo(myPPR.Value)) Dim pt1 As New Point2d(BasePt.X, BasePt.Y) Dim pt2 As New Point2d(myPPR.Value.X, myPPR.Value.Y) BaseRo = pt1.GetVectorTo(pt2).Angle End If Return SamplerStatus.OK End If End Function Protected Overrides Function Update() As Boolean myBRef.Rotation = BaseRo Return False End Function

I always rotate the block with bottom left point as fixed point. What can I do to make the block rotate around another point? such as centre point of the block?