First, arrow don't kill with knockdown, they kill by causing hemorrhage, death by blood loss. So, your broadhead is likely as important as your arrow, if not more so. Second, the 20" arrow will work. Yes, their trajectory will be steeper than a lighter 16" arrow, and they may stick out ahead of the bow, but they will work. Third, the length may not have anything with how they fly from your bow. Generally speaking, spine and FOC are more important than length. Arrow have to be stiff enough for your bow and 'front heavy' enough so the head steers the arrow and the vanes just stabilize it. Lastly, even 2" groups at 20 are more dispersion than I like; 3-4'' (from bench-rested bow) is unacceptable (for me). If you're not cocking with a rope cocker, by all means do so. That consistently centers the string and improves accuracy.