You should be ok balanced on center of wing tube (wings off). Most shifted their CG even farther back after getting more flight time and comfortable. My final CG eventually ended up aft of the wing tube, not exactly sure how far because I just kept moving the batteries back on the velcro. I do remember my ECU battery eventually ending up about middle of the equipment tray. Below is an old link on this topic...

http://www.rcuniverse.com/forum/rc-j...-settings.html