Roger,
I would agree with Bill. I went through several of my books looking at the oct-rd transitions and they were all over the place, behind, a head, Lined up perfect at the rod pipe. I did notice that the real long barrels tended to have the transiton further a head of the entry pipe. My Kentucy fowler has a 44" barrel and the transiton is at 15" and the entry pipe is at like 12'. I have a nice original 1750's French fowler that has a 36" barrel and the transition is at the step of the entry pipe. I dont remember how far down it is from the breach, its out on loan currently.