I'd agree fully that 25% is way too far forward. Hopefully someone that flew a Viking back in the day will chime in with a proper number. In the meantime perhaps run the model dimensions into one or more of the online CG calculators to get a better idea.
Generally for contest gliders you'll want to run the calculations at a little lower range of stability margin. I'd run the calculations for a 7% and a 2% stability margin to get a good idea of the most forward balance and the most rearward and use that as the range of optional balance. The final location is best determined from flying. Rusty old timers or lower hour pilots will want a little more stability at first until they sharpen up as the air time rises. All in all the further back the balance the more efficient the glider is. But there's a trade off where you reach a balance between airframe efficiency with the rearward balance point and the ability of the pilot to fly it smoothly and well when it's just a dot in the distance. Some amount is still wanted to aid on that.
Or the new fangled auto stability modules with fancy algorithms can be employed on a model that is set up to be much closer to neutrally stable.