I'm not sure that this applies to all locks but most of the ones I've worked with actually have a very small area that is IMO suitable for the lock bolt.
I'm looking at a Siler Percussion lock from the inside towards the outside.
All along the top of the lock is the bolster which stands proud of the rest of the lock plate.
Measuring this bolster right ahead of the screw that holds the bridle in place it is about .250 thick (top to bottom).
This thickness increases as one moves foward towards the location for the drum.
On this particular lock I wanted the lock screw as far aft as possible to prevent interference with the rear of the barrel but far enough forward so that there would be sufficient material around the screw hole.
I located this screw hole about 3/16 of an inch forward of the bridle (or 3/8 of an inch forward of the bridle screws centerline).
I tried to balance the location of the hole in the up/down direction from the sides of the bolster so, as the bolster is .250 thick in this area, I located the hole about .125 above the lower edge.
Once this location is found, I, like Birdog6 then determine the effects of this location on the opposite side of the gun. If it will position the sideplate on the side opposite the lock in a good place (not too high) I consider my work half done.
Because of the need to assure adaquate material around the lock bolt hole I locate it first. Then with the lock clamped I mark and center punch the holes location and then drill it thru with the proper tap drill.
Assuming the sideplate location will work, after drilling the lock, I place the lock back into the stock and using the new hole as a drill bushing, I drill from the outside of the lock into and thru the stock towards the sideplate panel.
I have done this with the barrel in place, and with it removed. In either case, sooner or later your usually going to have to drill thru the thick base of the tang.
This doesn't add any strength to anything. It is just a matter of fact that the damn thing is always in the road and the bolt or screw is going to have to go thru it. By the way, occasionally the screw location will place the drill so it just nicks the breech plugs base. If this happens, don't try to drill it the rest of the way thru. The partial or interrupted cut will break the drill.
You can always go back and file out a clearance notch in the rear of the breech plugs tang base so for now, just remove the barrel and finish drilling the hole for the screw.
You should enlarge the hole thru the tang base so there is plenty of clearance for the screw later.
Remember, this is just the way I do it. There are a lot of ideas from other members that work just as well. Perhaps even better.