I believe on many wings 25 percent is considered the super safe starting point. It will give you a positive nose heavy place to start... when you aren't sure where the cg is.
Then you start moving the cg back to where it makes the plane slow down and land best .. without becoming tail heavy... If you go to far it can make for a white knuckle flight !
A friend of mine had the same trainer you do, it flew great.. the fuse is so blocky and strong we called it "THE FLYING 4 by 4".