Bingo -- low wing member of the Ace/Doc Mathews 4-40 family, yes?

They've all got the same straight rectangular wing, so you can simply use the CG rule of thumb: CG belongs between 1/4 and 1/3 of the chord length back from the leading edge. For a plane I'm not yet familiar with, I always start at 28% and adjust from there.

There is an easy, general process for figuring out balance point on any wing... you have to find the mean aerodynamic chord... and then set the CG the standard 25-33% back from the LE at the mean chord. Not hard to do, but I won't belabor the details unless somebody need them...