diff --git a/chapter04/aboutsbus.xml b/chapter04/aboutsbus.xml
index 1928423ea..eeef03a3f 100644
--- a/chapter04/aboutsbus.xml
+++ b/chapter04/aboutsbus.xml
@@ -36,6 +36,14 @@
   to give an estimate of how long it might take to install a package, but the
   numbers can vary by as much as dozens of minutes in some cases.</para>
 
+  <note>
+    <para>In the cross edition, the SBUs are kept same as the original LFS
+    book.  They should only be considered as a reference. It obviously does
+    not make sense to predict the building time in Chapter 7 or 8 with
+    <quote>1 SBU</quote> measured in Chapter 5, as the target machine and
+    the machine running the host distro may be completely different.</para>
+  </note>
+
   <note>
     <para>For many modern systems with multiple processors (or cores) the
     compilation time for a package can be reduced by performing a "parallel