I weigh my ingredients, so I don't know how much 12 cups of sugar is. That might be fine. I use 1.25 pounds of sugar per gallon in most of my wines made from Welch's juice. You want to use a hydrometer, and get an original gravity of 1.085-1.100, preferably around 1.095.
That seems like too much grape tannin- I'd cut that in half, or not use it all, until you taste the wine after it's done and see if it needs it. Tannin provides "bite" to wine, but you may not need it in that recipe, especially not 1/4 tsp in four gallons. Same with the acid blend- 8 teaspoons seems like an awful lot. I'd use about 2 teaspoons, and add more later if the wine needs more acid.
You only need one package of yeast, no need to use two packages.
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