Thank you guys for the kudos....I'm just glad you enjoy it.
Tim your questions are good ones. Probably much better than my answers.
Maybe "straight line" rather than "center" makes more sense to you.
Mike if one has a template then this is not really needed.
I mentioned that and that I had a template and used it to router the pocket and thought it would help to show it. I also noted the difference should one not have a template-
After lining up the neck then mark on the body how long the tenon will be going.
What I didn't mention and why you asked (good question) was - after marking the length then one would put a stop block lined up so the router would stop exactly where it should.
With that one would have one of two things to do. Chisel the corners square for the
tenon or round off the corners of the tenon to fit the size of router bit used.