Start with the hole placed 1/8" ahead of the plug to the back side of the hole. This gives space for crud to collect before your TH becomes blocked, if you expect to have to shoot several shots before cleaning the barrel.
TH can be 1/16"( .0625"). Some are drilled a bit smaller. If you plan to shoot 3Fg powder in the barrel, start with the 1/16"TH. It should perform flawlessly for you. Center the hole .030" above the top edge of the pan. :thumbsup:
You may have to remove metal from the front of the plug( shorten the plug) and from the rear of the barrel, to make these measurements fit your LOCK, but the manufacturers of both plugs and barrels leave extra metal so that these parts can accommodate a variety of Flintlocks. As long as you have three complete threads on the plug, the plug/barrel will take any pressure you can create in the barrel. With today's modern fine threads being the norm, you are most unlikely to end of reducing the number of threads on the plug, by shortening it, to less than 6 full threads.
[For Example: a common plug size is 5/8 x 18 pitch, meaning that there are 18 threads on the shaft per inch. Most such plugs come with the plug about 3/4" long, or with 12 threads visible. If you had to reduce the length of the plug to 1/2" to accommodate a small lock, you still would have 6 threads visible on that shortened plug- more than enough for any pressure concerns.]
When you are working with a pre-carved stock, with the stock already inletted for a lock, the position of the lock inlet, and its size, pretty well dictate what size lock you can use in that STOCK.
The lock used has a certain length, so that it will control where on the barrel you can position the TH, to center it above your flash pan. If the plug is too long or too short, the lock still dictates where the hole will be drilled.
Since most plugs are made longer than needed for most locks, you can file the face of the plug to shorten it, and then file the rear of the barrel so that the new plug length butts against the back of the bore. Its sounds more complicated than it is. Just get the order in which you do your work set in the correct order, and you avoid future problems, with things like barrel lugs positions, sight positions, Trigger and Trigger Guard positions, decisions on Length of Pull of the stock, and where to locate the butt plate, etc. :thumbsup: