The C/G is determined by the mean aerodynamic cord (MAC). The MAC is the average cord dimension of the wing. If the wing is a simple straight, non tapered wing then measure the wing from the front of the leading edge to the rear of the trailing edge and that is the wing cord. 25-33% of that cord dimension (MAC) is the C/G range that is normally used (aft of the leading edge). If the wing is tapered, measure the widest part (usually at the root by the fuselage), then measure the narrowist part (that would be outward just before the tip) and average those two numbers to get the MAC for a tapered wing. The percentage number within that 25-33% range that you pick as your C/G spot to balance at is influenced by the type of aircraft, it's intended flying style, and the experience of the pilot. It is generally accepted that if you go outside the 25-33% area you may not have a flyable airplane. Yes, some hot dogs go past the 33% (in a tail heavy direction) with some aerobatic planes but they are not flying in the normal range and will either be very skilled or cleaning up a crashed airplane. Maybe both. For beginners, stay close to the 25% end of the range for a friendly flying airplane.