Since your lock is already inletted (mostly), I would recommend that you get your lock in its final position, since you will have almost no room to move it fore or aft. Then determine where your touch hole location will be on your barrel. (Your breach plug will need to be installed.) If you are using a touch hole liner, you’ll have to allow clearance so that it does not interfere with the breach plug. Now determine the required barrel location to match up the pan on the lock to the touch hole.
If you don’t have a book to refer to, I would recommend getting one. I like “Recreating the American Longrifle” by Buchele. I have read mine probably a dozen times. The best advise I ever received was to take my time. If you make a mistake, take a break, and research a potential solution. Then go back to it in a day or two when you are fresh. If you get stuck, ask here. There are plenty of experienced builders who will help.