Start with 4 inches as in the manual and go back SLOWLY; mine is set within 1/2 in back of the starting 4 in. If the CG is too far back, the aircraft will be real fun to fly but when will come the landing, you will also have fun not over controlling elevator because it will balloon very much.

When I was trying to find the CG I wanted, my battery pack was not held firmly in the fuse and came loose ---> it came all the way back to the end of the wire. I had a blast during this flight but I almost crashed the plane during landing because the plane was not just ballooning, it was snapping as soon as I touched the elevator (my elevator is real small compared to yours so be careful).

In short, get the CG as far back as your piloting skills allow you to land your plane safely.