For a trainer, 1/3 is the outer limit I'd personally consider... I like to set trainers up at about 28% and move it back as the comfort level grows.

A warning: Andy's right -- you sure can find a lot of advice about balancing your plane on the forums... Consider the advice about relative positions (like when somebody tells you to put it some portion of the way back the chord length). Unless the measurement came from your particular plane, don't listen to advice about position with respect to the wing spar (like when somebody tells you that most planes balance right at or a quarter inch in front of the spar)... a designer can specify that spar at 25%, 35%, 3" or wherever he wants, and then the builder can MOVE it wherever HE wants... No guarantee of any particular relationship between the spar position and the desired balance point of the plane.