Fixed up delta note

doc/src/fix_numdiff.rst

@@ -52,18 +52,19 @@ by two times *Delta*.
    It is important to choose a suitable value for delta, the magnitude of
    atom displacements that are used to generate finite difference
    approximations to the exact forces.  For typical systems, a value in
-   the range 1e-xxx to 1e-yyy will probably work well.  However, the
-   best value will depend on a multitude of factors including 
-   the stiffness of the interatomic potential,the
-   thermodynamic state of the material being probed, and so on. The only
-   way to be sure that you have made a good choice is to do a
-   sensitivity study on a representative atomic configuration, sweeping 
-   over a wide range of values of delta.  If delta is too small, the
-   output forces will vary erratically due to truncation effects. If
-   delta is increased beyond a certain point, the output forces will
-   start to vary smoothly with delta, due to growing contributions from
-   higher order derivatives. In between these two limits, the numerical
-   force values should be largely independent of delta.
+   the range of 1 part in 1e4 to 1e5 of the typical separation distance
+   between atoms in the liquid or solid state will be sufficient.
+   However, the best value will depend on a multitude of factors
+   including the stiffness of the interatomic potential, the thermodynamic
+   state of the material being probed, and so on. The only way to be sure
+   that you have made a good choice is to do a sensitivity study on a
+   representative atomic configuration, sweeping over a wide range of
+   values of delta.  If delta is too small, the output forces will vary
+   erratically due to truncation effects. If delta is increased beyond a
+   certain point, the output forces will start to vary smoothly with
+   delta, due to growing contributions from higher order derivatives. In
+   between these two limits, the numerical force values should be largely
+   independent of delta.
 
 .. note::