Well, your panel assumes the size of the screen, if there's not clipping area defined, so you can base your things on that. Remember to check offset if you want to more the panel around, or things will stay anchored to the screen even though you move the transform.
Each widget has a dropdown anchor where you can set each side of the widget to anchor to a side of "something else", that can be a Widget, a panel or anything else that derives from UIRect.