Usagi,
Before I got a Paterson orbital I used divided trays. I often got overdeveloped edges probably due to over-enthusiastic rocking..One day I found there were a few dead flies floating in the water that I had left in a tray. Out of curiosity I decided to find out how little I could move the tray and still get the flies to move around the tray. I found it was very little indeed. Slowly lifting one corner of the tray no more than 5 millimetres caused enough agitation to do the trick. So this has been my method ever since. I have the Orbital on a flat surface, and place a finger under a corner and slowly raise and lower it a few millimetres. I may repeat this once, or twice. Or may do it just once, before moving on to another corner -opposite or adjacent. I follow no pattern, but keep the developer moving slowly and continuously.
Once I tried vigorous agitation for five seconds every half minute -with nothing in between. I got tide marks of uneven development across the negatives. So I went back to continuous agitation. The tide marks were on 5x7 film, and the developer was D76 1+2. Don't know if Rodinal would have done the same.

Alan