It's really very simple,
make sure the fuselage is
level and the stabelizer is
level. Make sure the ends
of the stabelizer are the
same distance to a center
point near the front of the
plane. A tap measure works
fine for that.
You'll notice I'm using a
good old fasion buble level.
I'm not a big believer in
the popular laser levels you
see so often these days.
The reason I don't like them
is because they are usually
short. The quality of the
level is in large part due
to the lenght of material
contacting the surface you
are trying to level.
Imperfections in the surface
will through a short level
way off, but not a long one.
You can easily get within a
64th of an inch (about .015)
using a good buble level.