So, you have 0.91 * 6.7 ounces = 6.097 ounces of pure alcohol.
You want to have a solution with 70% alcohol, so, 6.097 = 0.7 * x, therefore the, x = 8.71
So you should add 8.71 - 6.7 = 2.01 ounces of water to get there.
That's atleast what the maths says, I don't know chemistry so if...