Text over image with matching relative positioning

i'm in stuck. The problem is how to place text on a static image and keep position depends on it between different screen dimensions. To achive this i've tried the layout above:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://ift.tt/nIICcg"
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="@android:color/black"
    android:padding="@dimen/default_view_padding">

    <ImageView
        android:id="@+id/iv_background"
        android:layout_width="500dp"
        android:adjustViewBounds="true"
        android:layout_height="900dp"
        android:layout_centerInParent="true"
        android:src="@drawable/example"/>

    <TextView
        android:layout_alignLeft="@id/iv_background"
        android:layout_alignStart="@id/iv_background"
        android:layout_alignEnd="@id/iv_background"
        android:layout_alignRight="@id/iv_background"
        android:layout_alignTop="@id/iv_background"
        android:layout_width="wrap_content"
        android:layout_marginTop="190dp"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:gravity="center"
        android:text="Dummy text string"/>

</RelativeLayout>

But on different screens it looks differently. So, by example, screenshots. The static image background (in example white image with green line), the view with text on nexus5, on nexus7. As you can see, a text placed on different places over image. I doesn't know why is it happend, becouse i'm using dp and relative layout.

I tried wrap_content/match_parent on image sizes, without ajust view bounds etc. And it haven't help.

Ty for answers.

EDIT: I want the text to always be above the green line on the same distance in different screen dimensions.