Yeah, Void would be a little bit better than Mystic in an otherwise maxed-gear ideal condition. But if that's the case, you can probably use Ahrim's or something better than Mystic anyway. :P But any conditions lower than that, and Mystic would be better. The +8 is a constant. Without it, you would have 0% accuracy if your Attack/Magic/Ranged (respective) stat were at 0, regardless of what weapon you're using. Unrelated, but you can still get 0% accuracy if you get -64 accuracy in your style. Take level, multiply prayer bonus and round down, add 8, add style bonus (+3 if Accurate stance), multiply by void bonus and round down. So it would be ((1.15 * Level) + 8) * 1.3