I think the best way to not lose your shot line is to forget about it. Once you have the line sighted perfect while standing up, flip your eyes to the CB and forget about the OB/line/pocket. The only thing that matters is to not lose the 180deg view of the CB facing you. Call it edge-lock or whatever--- never lose sight of the exact right edge and left edge of the CB (meaning, if you eyes move a hair right, you lose the left edge initially visible).
When you lock into the CB and never lose sight of the visible 180deg, nothing else matters in transition but NOT losing it. If you never lose that exact sight, center ball is your shot solution once you address the CB.
I think being exact with not losing the CB face (edges) is far easier than being perfect in a physical motion while locking on a shot line. If you never lose the CB, your body will follow your eyes.... and that makes for a perfect transition.