If you can't get the plane to "balance" - that is, to stay on the balancer without teetering to one side or the other, it's no big deal.

If you place it on the balancer and hold it level and it will drop to one side consistantly (Nose or tail) add weight to the other side until it will drop to either side equally.

In other words, let's say you place it on the balancer, level it and it drops to the tail, now level it again, but tip it slightly nose-down and it tips toward the nose, you are close enough.

Remember that the balance point is not necessarily an EXACT point. Adding fuel will change it, getting dirt and oil on the plane will change it, etc. So getting it close is close enough.

This is why we usually tell people to err on the nose-heavy side. This way, if there are any minor changes, you have a built-in safety margin.