Your idea of making the blocks with the divot so the starting point is centered, but you do have to make the cherry first - to the correct ball size that fits the rifle. It is made from hardenable steel, annealed. then the cherry has to be filed (as shown in the picture above) to be able to cut the soft iron or aluminum mold blocks. It is not an easy task, and if your cherry is off, your ball will wobble. Good luck.