What can happen is that the engine may begin to suffer from fuel starvation if the tank is to far way... the pressure from the muffler just isn't enough to force the fuel that extra distance from the tank to the carb during vertical flight for example. A pump or pressure regulator can solve that problem but... keep it simple as they say.I'd set the tank closer to the firewall for now, then try moving it back some as you go along.You're going to get slight trim changes because of fuel usage, just wouldn't bother yourself about that at this stage or certainly if you're only planning on using the model for sports fun.If it starts driving you crazy, mount the tank on the CG and use a pump.What I've done in the past is to set a couple of simple flight conditions on the tranny, flick the switch to get a slightly different trim condition as the fuel is used... reached a conclusion...what's the point!To much work... and the trim change isn't that much, IC sports aircraft have been set-up that way for decades, hasn't ruined anyones funJust fly and enjoy.