If I understand correctly, you fermented the must for 5 days until the specific gravity got down to 1.012 and now you're going to add yeast and some sugar and you want to know how much sugar to add? If you had taken the specific gravity soon after the grapes were crushed, it would have helped. You can take a guess at what the sugar content of the must was when the grapes were crushed and then add an amount that, together with the original sugar content would give you, say 12% alcohol. You might get help here in making that guess about the grapes from someone familiar with that variety. But apparently you don't know the variety or it's a secret or something. Well, just guess they're about 15 or 20 % sugar. You saw the grapes and could have tasted them -
Never mind all the figuring and estimating I was going to do. Add sugar dissolved in water - 2 pounds of sugar stirred into 1 pound of hot water on the stove until you bring the sg of the must up to 1.085 or higher. This could be in the neighborhood of 12 to 16 pounds of sugar. - No, wait, the must could have had a lot of natural sugar to start with and this could get you a stuck fermentation or some 18% alcohol. - Oh, what the heck, learn a lesson, do it. What does OG mean?