On all of the guns I build, when it comes time to drill for the rear lock bolt, I centre punch the bolster in the lowest and rearmost point in the bolster that will support the screw and not run out through the side. It would be easier to show this with a photo but I'm having trouble finding one, so pls excuse my description. In other words, as far to the rear and as low as you dare. Drill the tap drill hole in the lock plate first. Re-install the lock plate in the lock's mortise, set up the stock in a vise on your drill press table so it is square with the drill bit and drill through the hole in the plate and the wood with the tap drill. Remove the lock plate from the wood. With a clearance drill bit in a hand drill, enlarge the hole in the wood so a tap will pass. Re-install the lock plate in the wood, and tap from the offside through the wood into the lock plate. Turn the tap 1/3 turn, and reverse to break the chip. Continue tapping like this until the tap emerges through the lock plate. Remove the plate, clear away the chips on the inside of the mortise.
After you have finished the offside's surface, and have filed the edges of the lock washer with a slight draft, pass the lock screw through the washer and the wood into the lock plate and turn it down securely. Now trace around the washer with a sharp pencil, remove the screw and the washer and inlet the washer/escutcheon.
This sequence of work has worked for me for close to 300 muzzleloading guns.
The front screw hole is done a little differently. When you were inletting the lock plate and especially when creating your blueprint to ensure all of the parts lined up correctly, I hope you positioned the lock plate so that the forward screw would come through the plate in the centre of the forward boss of the plate and pass beneath the barrel without interferring with the rod hole. Either way, by measuring the barrel and plotting it and the web between your barrel and the rod hole on the offside of the stock, you can determine where the lock bolt must pass. Mark it's position on the offside panel and centre punch it gently. If you are confident that the screw can pass through the stock beneath the barrel, through the web, and into the centre of the plate's forward boss, you can mark and centre punch the lock plate where you want the forward screw to go. Then drill it with the tap drill. Re-install the plate into the mortise, secure the gun in the drill press vise so that it is square with the TAP drill bit, and go ahead and drill through the hole in the plate, the wood and out the offside of the wood. Inlet the escutcheon the same way you did the rear one.
That's a lot of words that a short film or photos would have described more effectively. Go slow, use common sense, and enjoy the process.