To make certain prongs are down tight on the crown of the stone, hold
the stone eye level. Then, hold your index finger about 1" above the
stone. This will shield direct light from the stone, causing a
silhouette of the stone and prong. Then, you can easily see light
coming through any gaps between the stone and prong. If necessary,
tape a 3 X 5 white index card on the wall in front of your bench to
provide a light background to hold the stone in front of.
Brad Simon CMBJ