Mike is right, get some books if you don’t have any and see how they did it.
Here’s what I was taught regarding placement of the lock, in priority order:
1. Pan to touch hole alignment.
2. Architecture of the nose and tail of the lock relative to the wrist and the fore-end.
3. (And in last place...). Relationship of the forward lock bolt to the web.
In other words, place the pan in the right place reference to the touch hole first. Then play with rotating the tail of the lock to get the architecture right. Remember that the bottom of the barrel can be notched to allow passage of the forward lock bolt (within reason, given the barrel wall thickness).
This is quite often seen on original guns, showing that they also used this priority order. I have an original Berks County gun with a notch in the barrel for the lock bolt, and a remnant Barrel from about 1805 -ish with a notch for the lock bolt.
Also, the forward lock bolt can be notched to allow the ramrod to pass below it, or the ramrod can be tapered in the last couple inches to pass by the lock bolt.
The important thing here is that, within reason, the front lock bolt placement is not nearly your first or second concern because you have some freedom of movement with it. It does not necessarily have to be centered in the web if you use the aforementioned fixes, just as the old guys did.
The advice I gave above is generic, and my knowledge of trade guns is roughly kindergarten equivalent, so I would say again, look at examples of how they built the specific style you are building.
Hope this helps,
Norm.