The main spar is the heavy support "stick" in the wing that runs from wingtip to wingtip.

25% MAC CG starting point is a known forward safe starting point... almost always you will find that to be noseheavy when flight testing.

See the thread about doing aerobatics with a Hobbicl SuperStar for more about "playing with" the CG, and re-trimming an airplane for higher performance. (when you no longer really need a trainer...)