I also disagree with Bigdaddy. If lateral balance is out enough you can't do a simple manuever like a loop. The aircraft will want to roll out of the loop to the heavy wing.
I do believe in CG machines, a pencil is a very small area to put the weight of your aircraft on. Possible to send the pencils spearheading through the balsa sheeting.
Try not to add weight, instead move your battery and reciever around. if all else fails then add weight. I will cut open the monokote and use stick on weights then dab drops of epoxy to help hold the weights in place, if nose heavy. If tail heavy I'll put the weights around the fuel tank.
For lateral balance I cut the monokote open at the wingtip, and place a weight on the cg line on a rib or spar and secure with drops of epoxy.