If you're happy with the flavour and balance you can certainly finish the wine off at that SG. Just make sure to stabilize the wine adequately with sulphite and sorbate to prevent any refermentation in bottle. Since you don't know the starting SG the only real issue is whether the alcohol level might be too low to keep the wine stable... alcohol is a natural stabilizer, and if it's too low, you run the risk of bacterial spoilage, etc. That being said, some sweeter German Rieslings have an alcohol content as low as 7%. I would probably increase the amount of sulphite a bit just to be on the safe side.