LW: I think you're overthinking this. Lay a steel scale or even a slat of wood on the stock, eye ball it up so it looks as square as you can make it, and with the other hand and a pencil, draw a line across the wood. It won't be straight likely, because of the curve of the wrist, but it doesn't matter. Don't cut in at the line, just use it as a stop spot when moving the barrel backward.
Put transfer colour on the butt end of the breech without the plug in, lay it in the channel, and bump it on the muzzle. Remove the barrel without tipping it up at the front end, and remove the colour transferred to the wood. Repeat until you reach your mark, but continuously check other indicators, like where the centre of the pan and the vent hole will be. This process is slow and requires patience, so little by little, you move the barrel back until it suits you. One doesn't cut all of the wood out at once. That would create problems along the sides of the barrel, creating gaps.
