Recently I needed to have percentage based layout (I wanted two ListViews side-by-side at 50% of the screen each). Below is an way to achieve it using LinearLayout with layout_weight:
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal" >
<ListView
android:id="@+id/lstPrev"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight=".50" />
<ListView
android:id="@+id/lstCur"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_weight=".50" />
</LinearLayout>
Notice that both ListViews have layout_width set to 0 and layout_weight set to .50 (for 50%)