That's right, Robert, I had forgotten that the interval generated by the app is from the right outside of the first object to the left outside of the second. You would think it would be center to center, or at least I would think that. It's not always that important but with this it can really mess you up.
However, this project I created I had done the old fashioned way 75mm, 150mm, 225mm... and so on.
Thanks for the reminder I need to keep that in mind for other projects.