I would do a bench trial with a measured amount of the wine say a 1/4 cup and then add the grape juice a 1/4 tsp at a time util you reach the desired level. best is to run about four sample add 1/4 tsp to first, two 1/4 tsp to second, thee 1/tsp to third etc. do taste test, doesn't work continue with new samples. once done calculate addition to main batch at potassium sorbate and k-meta to insure fermentation does not restart