When I inlet a lock, although the vent is not drilled, it is marked with a centre punch. It is no trick to inlet the plate exactly to coincide with the mark. But it takes careful layout. With the lock stripped, and the plate prepared with filed draft, lay the bolster on the stock to align with the vent. Using a sharp pencil, draw in the bolster, and then inlet it. You obviously cannot get the bottom line, but you can find it having marked both ends and the top. Once the plate is laying flat on the wood with the pan aligned with the vent, simply inlet the rest of the plate. Then adding the guts, starting with the bridle....etc.