RE: CMP Sukhoi SU-31 90 69" CG Help
As with most ARF's start with the C.G. at 25% of the wing chord length at the LE of the wing at the root.
The plane flies fine this way. You can move it slowly back for better response after you've determined how you like to fly it.
CMP planes are similiarly built so you can merely use the manual for a more recent model as a guideline.