I think you are on target. We have used a similar technique for our three boxers in the past and are currently training Layla this way. Let me add a few thoughts.
Two of ours ended up being quite good off leash. They would stay near or return on command, we could walk trails without concern, and they would know the boundaries of our property as if it was an invisible fence. On the other hand, Lucy would simply drift off at her own desire and never look back. (Real frustrating) As we know, boxers have their own personalities.
Even with the two who were good, we would never have them off leash unless it was safe. You never know what might happen.
Same as the 20 foot leash technique, we practice and play with them while on a long loose rope in large fenced in areas, eg football fields, from early on. If nothing else they learn to stay in the vicinity as we walk around. We use "wait" (don't go farther), "here" (get closer but not a recall), "let's go" commands as well as "come,"
It can be hard to do, but to get them to follow you, try running the opposite direction. Nothing like a chase to get their attention. BTW, this doesn't work all the time.
Regular obediance training is pretty much a must do if you are going to have a chance for off leash.
There is a concern to weakening the recall to "come", when they are off leash, so we avoid using it in situations we know they won't listen.
Currently trying to get Layla to help me do yard work while she is off leash, but with a 15 foot rope. I give her a ball and throw it occasionally, so she doesn't get too bored. Most of the time she is doing pretty good. When she does leave the yard, I get her and put her inside the house. It is clear she knows better, so I am hopeful she'll figure it out the ramifications of exploring. We'll see.
Good luck to anyone trying off leash. It is really nice if you can do it in safe areas.
. Brooke has excellent recall, always has but see a cat and zooooom!
