Ive read a few posts on Mikes forum regarding the showtime and I believe that he said the easiest way is to set the firewall vertical and set the incidences from that. When i did did ive gotten some way off readings upwards of +4 deg. When i mounted the motor and everything i leveled the plane via the crankshaft of the saito and seen much closer measurments. On my plane every one of my wings are off like .5 to 1 deg. Im going to fly it anyway and see what happens. Here is the link to one of Mikes threads on the Showtime.
http://www.rcuniverse.com/forum/m_38...tm.htm#3815874

You can try to level the plane via the firewall or just mount the motor and level it that way. Or the leveling the plane via the throttle servo tray. Which by the way is a good idea... Clever one!!