Mine is really twitchy at the factory marks. I measured out and marked 25 & 30% of the chord line along the bottom of wing. I found that with the model balanced at factory was extremely tail heavy. Mine flies really well at 25% with the nose down 3/8".
The center of gravity has to be placed under the thickest part of the wing. The factory marks are nowhere near there on any of my E-Starter wings. I read an excellent article on this in Backyard Flyer. Hope this helps.