A rough system that will give a forward CG (gets you flying and you can slowly shift the battery to trim to your taste):

For both wings...
Mark the 25% chord at the root.
Mark 25% chord at the tip.
Connect.
Measure 50% distance from root on that line.
The two points are the starting balance point.

Noseheavy every time... noseheavy flys sluggish. Tailheavy flies once.

Often, the spar is just aft of the lines. The spar is usually on the correct CG line.