Hi Pete,
I should have mentioned "overhead 64 mpu 84" only make sense in combination with a shaper limit (well, they will make sure the cake statistics will be more reflective of what is happening on the ethernet wire, but I am not sure whether that is worth the run-time cost the overhead calculation incurs).
Somewhat unrelated, I wondered about all the excitement about irtt and cloned the repository to my mac, and was absolutely delighted to realize that irtt will also effortlessly work under macos. I only have run the demo from the readme.md with both client and server running at the same machine, but I got results that look reasonable on first sight (but I admit I really do not know exact;y what to expect). This is really great!