With the 5 cm size reference for that small sharply tapered section.... I would say to balance it at 12 cm back at the root, or 5 cm back at where the sharp taper ends.

It is possible... that the 5 cm back reference is from the leading edge at the wingtip... which would be even further back... (and would make more sense to me because the LE of the Mustang wing should sweep back enough that the above could have the CG forward of the LE at the tip....)

Look up in the Aerodynamics forum discussions about "MAC" MAC is mean aerodynamic chord. You want to start with the CG close to 25% MAC. There is an easy way to get this measurment for anything other than an eliptical wing (like a Spitfire) And the drawng for the method has been posted many times in that forum.