how to move 20dip margin left from center?

i want to move a seekbar 20dip margin left from center.

android:progressDrawable="@drawable/progress_vertical"
            android:thumb="@drawable/seek_thumb" android:layout_height="80dip"
            android:layout_width="20dip" android:layout_marginBottom="50dip"
            android:layout_alignParentBottom="true" android:visibility="gone"
            android:layout_centerHorizontal="true" android:layout_marginLeft="20dip" /> 

but the above xml code shows it center only.