Add clarifying paragraph about SBUs.

git-svn-id: http://svn.linuxfromscratch.org/LFS/trunk/BOOK@3016 4aa44e1e-78dd-0310-a6d2-fbcd4c07a689

chapter02/aboutsbus.xml

@@ -16,15 +16,21 @@ statically linked Binutils in Chapter 5, and the time it takes to compile this
 package is what we call the "Static Binutils Unit" or "SBU". All other compile
 times will be expressed relative to this time.</para>
 
-<para>For example, the time it takes to build the static version of GCC is 3.9
-SBU. This means that if on your system it took 10 minutes to compile and
-install the static Binutils, then you know it will take nearly 40 minutes to
-build the static GCC. Fortunately, most build times are much shorter than the
-one of Binutils.</para>
+<para>For example, the time it takes to build the static version of GCC is
+&gcc-time-tools-pass1;s. This means that if on your system it took 10 minutes
+to compile and install the static Binutils, then you know it will take
+approximately 45 minutes to build the static GCC. Fortunately, most build times
+are much shorter than the one of Binutils.</para>
 
-<para>Note that SBUs don't work well for SMP-based machines. But if you're so
-lucky as to have multiple processors, chances are that your system is so fast
-that you don't mind.</para>
+<para>Note that if the system compiler on your host is GCC-2 based, the SBUs
+listed may end up being very inaccurate. This is because the SBU is based on
+the very first package, compiled with the old GCC, while the rest of the
+system is compiled with the newer GCC-&gcc-version; which is known to be
+approximately 30% slower.</para>
+
+<para>Also note that SBUs don't work well for SMP-based machines. But if you're
+so lucky as to have multiple processors, chances are that your system is so fast
+that you won't mind.</para>
 
 </sect1>