World Position to the specific Rect screen position

//Variables

GameObject targetObject;

Camera renderCamera;

RectTransform canvasRect;

//Update or somewhere

Vector3 viewportPos = renderCamera.WorldToScreenPoint(targetObject.transform.position);

//World Position to Viewport Position

Vector2 localScreenPos = new Vector2();

RectTransformUtility.ScreenPointToLocalPointInRectangle(canvasRect, viewportPos, renderCamera, out localScreenPos);

//Viewport Position to local Rect Position

참고: https://answers.unity.com/questions/799616/unity-46-beta-19-how-to-convert-from-world-space-t.html