Relative percentage based dimensions

constraintlayout_relative_percentage.xml

<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <Button
        android:id="@+id/button"
        android:layout_width="180dp"
        android:layout_height="wrap_content"
        android:background="@drawable/bg"
        android:text="Button"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Space
        android:id="@+id/anchor"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:visibility="gone"
        app:layout_constraintHorizontal_bias=".7"
        app:layout_constraintLeft_toLeftOf="@+id/button"
        app:layout_constraintRight_toRightOf="@+id/button" />

    <Button
        android:id="@+id/another_button"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:background="@drawable/bg_red"
        app:layout_constraintLeft_toLeftOf="@+id/button"
        app:layout_constraintRight_toRightOf="@+id/anchor"
        app:layout_constraintTop_toBottomOf="@+id/button" />

</android.support.constraint.ConstraintLayout>