Is there a way to fill parent minus a fixed number in Android RelativeLayout?

Put a android:layout_margin="10dp" on the ImageButton, along with the

android:layout_width="fill_parent"
android:layout_height="fill_parent"

1. Use the xml attribute android:layout_marginLeft to specifies extra
   space on the left side of your view.
2. Use the xml attribute android:layout_marginRight to specifies
   extra space on the right side of your view.
3. Use the xml attribute android:layout_marginTopto specifies extra
   space on the top side of your view.
4. Use the xml attribute android:layout_marginBottom to specifies
   extra space on the bottom side of your view.
5. Use the xml attribute android:layout_margin to specifies extra
   space on the left, top, right and bottom sides of your view.

In your case your ImageButton declaration look like this

<ImageButton
    android:id="@+id/button1"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    android:text="Button" 
    android:layout_marginLeft="15dp"
    android:layout_marginRight="15dp"
    android:src="@drawable/a"