Very good advice to start with more forward balance, no doubt. In this case, I wouldn't just start at 25% from leading edge. The "lifting stab" will not lift before the C/G is set far backwards, but its camber and especially the wing's camber make for a lot of nose-down pitch (airfoil moment). That's why such models have a lot of decalage (say 4° incidence difference between wing and stab) and a rearward C/G. This model has a very small elevator, which is undoubtedly enough if balance is OK but may be not powerful enough in case the C/G is very far forward.
The attached pictures show a glider I built in the sixties. It has similar configuration and undercambered wing airfoil. Unfortunately, the plan had to be repaired with tape but I drew a red arrow where the C/G should be. The instructions specify 69% from leading edge. Another well-known example is the Telemaster with (not even undercambered but) flat-bottom airfoil where the neutral point (aft limit of C/G) is at 50%. I would use at least that as a starting point.
