You can use this calculator to determine where your CG should be so there won't be any question.

http://adamone.rchomepage.com/cg_calc.htm

It's possible that the original owner just flew it nose heavy. It's also possible if there is some sweepback in the wings that you are measuring it wrong. If you really do need to move the CG back, I would make an extension for your battery pack and move it back to where it needs to be. It's a simple task to fly the plane with removable lead on the tail to verify the CG, then take it off and make a hatch for the battery.