How many gallons? Not "a" gallon, right? Beersmith tells me it's 4.4 gallons to get 1.070 with those ingredients. <edit, ok, saw your correction>
Anyway, why do you think it will get to 9.3 ABV? I get 6.8, assuming a "normal" attenuative yeast. To get to 9.3% you'd have to get it down to 1.000, which won't happen with the extract. By the chart, I assume you mean the "potential alcohol"? Ignore it, it's useless. It all depends on the attenuation you get from your yeast, assume 75% until you get some numbers to back it up.
At this point, I don't think you need a refractometer. The main advantage is the small sample you need for measuring gravity, you've got other things to spend your money on. Other than that and the speed of cooling a smaller sample, they are in no way better than a refractometer in my opinion.