As someone just said, this is very like chicken math. It's plant math, and some of us get it BAD in the spring!
Plant it where you want it. Sounds obvious, but once it's there, it. is. there.
I have had a spot where it didn't get enough of a start, and didn't grow. I had another that I dug up for a friend, and last I saw, one teeny-tiny little leaf was trying to make a comeback. And that was when I really was trying to get the entire root.
I have two other comfrey "areas." One is near the driving path up the hill, and I think the roots get disturbed enough that it's not ever going to grow as well as the other area. The other one has tripled in size.