As you're finding out there's no absolute location for the balance point. If you can't live with the trim change that occurs from a full to empty tank then move it. The only issue will be the engine's ability to draw the fuel. If it's got a proper pump on the engine you may be fine. If it doesn't have a pump and only relies on the weak muffler pressure idea then you're stuck with the tank being as close to the engine as practical.