I've been trying to set up a Domino-Line (parent object) with 5 Dominos in it (Children). When mousing over the Domino-Line, I want two UI-elements to show up: A '-' sign behind the first domino in line, and a '+' behind the last in line.

However, when trying to use WorldToScreenPoint, the UI-elements get placed all wrong, to the left of the entire line above eachother.

So how would I fix this?

EDIT: Here's part of the code, the OnGUI function

void OnGUI() {
    if (playScript.gamePlays == false) {
        if (mouseOver == true) {

            Vector3 screenPosBegin = Camera.main.WorldToScreenPoint(transform.TransformPoint(firstStone.transform.localPosition));
            Vector3 screenPosEnd = Camera.main.WorldToScreenPoint(transform.TransformPoint(lastStone.transform.localPosition));

            if (amountOfStones > minStones) {
                if (GUI.Button(new Rect(screenPosBegin.x, Camera.main.pixelHeight - screenPosBegin.y, 50 * (1.1f - (screenPosBegin.z/20)), 50 * (1.1f - (screenPosBegin.z/20))), detractStoneTex)) {
                    amountOfStones--;
                    Destroy(lastStone);

                    touchBox.size = new Vector3( (amountOfStones * 0.5f) - 0.5f, 0.6f, 0.4f);
                    touchBox.center = new Vector3( touchBox.size.x/2, 0, 0);

                }
            }


            if (amountOfStones < maxStones) {
                if (GUI.Button(new Rect(screenPosEnd.x, Camera.main.pixelHeight - screenPosEnd.y, 50 * (1.1f - (screenPosBegin.z/20)), 50 * (1.1f - (screenPosBegin.z/20))), addStoneTex)) {
                    amountOfStones++;
                    GameObject newStone = Instantiate( Resources.Load("Domino"), Vector3.zero, Quaternion.identity) as GameObject;
                    newStone.transform.parent = transform;
                    newStone.transform.localPosition = new Vector3((amountOfStones * 0.5f) - 0.5f, 0, 0);
                    newStone.transform.localRotation = Quaternion.identity;


                    touchBox.size = new Vector3( (amountOfStones * 0.5f) - 0.5f, 0.6f, 0.4f);
                    touchBox.center = new Vector3( touchBox.size.x/2, 0, 0);
                }
            }
        }
    }


}

Right now I've made it to look -sorta- ok, by subtracting the height from it.