Jeez......... somethings can be made much harder than they are.

The aft deck is the large flat area between the wing and stab. They show a clear picture of it on page 27. It's where you glue formers f4a and f5a. Place a small level on this horizontal sheet (the aft deck). Shim up the tail of the plane until this area is level. When you get the incident meter place it on the horizontal stab (see included instructions on how) and shim the front of the stab up until it reads +3 degrees and glue it into place.

Walaaa!! done. All the hard figuring is done for you. Now get back to work!