android layout margins with percentage

You can have invisible Views in your LinearLayouts as spacers and use the layout_weight mechanism to assign them relative size.

Example:

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_weight="1">

    <Thumbnail
        android:id="@+id/thumb1"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="4" />

    <View
        android:layout_width="0dp"
        android:layout_height="0dp"
        android:layout_weight="2"
        android:visibility="invisible"/>

    <Thumbnail
        android:id="@+id/thumb2"
        android:layout_width="0dp"
        android:layout_height="match_parent"
        android:layout_weight="4" />

</LinearLayout>