Keeping the rings of the mandala centered is crucial. Everything depends upon being within a pixel or maybe two of center so that we can maintain symmerty of design.
For me, the trickiest part is when I add my first new element to a ring and then duplicate it. I usually start at the top and rotate the first element 180° and then drop it to what I want to be the bottom of the ring. I will use my grids and design landmarks to get the new element into approximate position and then I will check my positioning by selecitng both elements in the move mode. This will give you a boundinng box around those two elements. Check that the central circle that goes with your bounding box lines up with the crossing of your central grid lines. Adjust and recheck as needed.
If you get this alignment precise, then adding additional layers to the ring is easy.