RE: Super Cub LP wont balance
If your buddy is "extremely experienced" in RC, then why don't he help you balance it?
Use the recommended COG that is in the manual, if you don't have the manual then get the correct COG at HorizonHobby.com for this plane.
You have a good balance checker so if it's tail heavy, add some nose weight, try to move the battery back and forth as much as possible to make COG Changes, even cutting away some foam to move the battery more is sometimes helpful, but if that doesn't work, then you will have to use weights.