Every UIRect (widget, panel) has localCorners and worldCorners, whichever you prefer to use. I'd suggest you take the local corners and then Vector3.Lerp between the values to get what you want. Don't forget to adjust by the slider's transform though. If you want it to be in local coordinates for positioning of another widget, use worldCorners, calculate the two points you need, then yourWidgetThatYouWantPositioned.transform.parent.InverseTransform(point) to bring it into local space.