I just took a better look at the instructions.

It looks like the stab saddle(the part you glue the stab to) will set the proper incident when the horizontal stab is glued down. If you look at the rear portion of the tail you will see the angle of the stab saddle is slight at a upward angle compared to the aft deck. So don't sweat it, just glue it down but make sure it's lined up straight as the instructions say.

Hope this helps. Sometimes the simple solution is the best solution.