I see a lot of people talking about a fuel tank... This plane is an electric powered glider. So ballance this plane fully loaded, ie. with the battery installed. Placing the CG to the forward limit will give a faster glider with less glide time without thermal activity. Ballancing toward the aft limit will decrease your glide slope and and slow the plane down giving you longer flights. As a newbee I recomend the forward CG mark for you untill you become more comfortable with the plane. As you move the CG aft you will find the plane will be more sensitive (responsive) to control inputs and it will stall more quickly. Good luck. Have fun. Never give up.