Hi petergomes,
tank empty c/g level or nose down a little!! Ruler or something similar works fine,but in flight other forces effect the plane. Tork of the prop,propwash on the plane,thrust line, just to name a few items,so in flight you will probably have the controls trimmed a little off center,not to worry,plane should be trimmed to fly level with no control inputs as you change speed or even props, the plane can fly different,trim the plane at the ave.speed you fly and the prop you usually use,as you have already learned,c/g changes as weight of fuel changes in flight,hope this helps, John.