The regenerate button seems to cycle (inconsistently?) thru the grain sizes. Undesirable; if I pick a grain size and regenerate, I'm looking for a suitable arrangement at that size.
The "refresh" button randomises as much as it can, but you can prevent a section from being randomised by "locking" it (clicking on the tiny lock icon).
So you can pick a size, lock it and hit "refresh" as much as you wish. I'm glad that you like the app, cheers!
Otherwise, nifty!