Hi neuroza
The front of the wing is called the leading edge (LE). The back of the wing, including the ailerons, is called the trailing edge (TE). Your wing cord is 300MM. 25% of that is 75MM. Measure that from the wing LE toward the wing TE. That is the location of the CG. After you fly the plane you may want to move the CG a little to make the plane do what you want.