Not to worry about the tank. Unless you have an engine with a fuel pump, you want the tank as close as possible to the engine to maintain a consistent fuel flow to the engine, and smooth engine performance.

With regard to the CG, the reason you balance with an empty tank is because a forward CG is more stable than one moved towards the tail. So, if you balance on an empty tank, you'll be stable as the fuel runs low and more stable when you start out full. Now if you were into aerobatics with a high performance plane, you might wish to use a fuel pump and center the tank over the CG to maintain a constant balance and flight characteristics. But on a trainer and all but the most demanding applications, this isn't necessary.

So, install it according to the directions and enjoy.

Good luck,
Heath