Balance on the UCD 60 can be an interesting game of trail and error....

Mine has the tank on the CG and is balanced at 6" and I like it there personally.

If you have tank mounted on the CG 6" will be slightly pitch unstable but nothing you can't deal with, simply put, where you point the nose is where the plane goes AT ALL TIMES AND AIRSPEEDS. This gets newbies into trouble when they try to flair on final to high and stall the plane, you have to FLY it to the ground. At 6.25 it's even worse and I didn't like it there. Be aware that getting one of these on the ground in the wind can be an exercise in frustration if your back past the 6" mark and have the tank on the CG, the plane will want to nose up and keep flying, gets interesting and a bit nerve wracking at times.

Anything ahead of the 6" mark, or if you have the tank in the nose(even at 6" ) will be far more docile and the plane flies like a big tainer IMO, very easy and predictable.