What MinnFlyer said. The plans or instructions will give the optimum C.G. point, or an acceptable range. Most trainers are 23% to 25% from the leading edge, but that's not chiseled in stone. Swept wings, tapered wings, high wing, low wing (balance the model upside-down), two wings. All different. It should be on the plans or in the manual.

I prefer two pencils with erasers to my fingers as the pivot point is then stiffer and smaller. Finger pads are too wide and forgiving. But it's just a "ball parky" starting point anyway. How he model flies and reacts is how to balance her out.

Put in everything you will be flying with except the fuel. I even pre-balance before covering fore-and-aft and side-to-side (strings around prop shaft and rudder hinges with model hung from ceiling) and add lead is necessary so it can be the smallest amount needed under the covering at the wingtips or vertical fin root/fuselage tail. Lighter always flies better.

With ARFs I have come to rely on cents or flattened lead pieces taped to servos in the wings or fuselage for balancing if battery placement alone won't do it. Easy access and well secured.