A pair of dividers is all you need. To mark the center line, set the
dividers close to half the width of the ring, then scribe the centre
line by running one leg of the dividers along the edge of the ring
and repeat from the opposite edge of the ring. You should then have
two lines close to the centre of the ring. The exact centre is
between the two lines.
For stones that don’t go all around the ring, lay the stones on a
flat surface and space them apart to your liking. Measure the
distance between the centres of the two end stones and divide that by
the number of spaces - not the number of stones. Set your dividers
and step off the stone centres around the ring.
If the stones go all around the ring then draw a “star” on a piece
of paper. Sit the ring centered on the star and mark off the centres.
Drilling the actual holes and cutting the stone seats so they follow
your markings is a bigger problem than the marking out. Forget about
getting each hole perfectly centred first time - unless you have some
exotic machine tools. With a sharp point make dots where you want to
drill, then use a small centre punch to enlarge the dots. Drill
through with a small drill, for example if the stones are 3mm then
drill with a 1mm drill. Next enlarge the holes with a 1.5mm ball burr
making any corrections that you see are necessary. Repeat with a 2mm
ball burr - making further corrections, and again with a 2.5mm ball
burr keeping a close watch on how the hole is centered in the width
of the ring, and also to make equal spaces between each hole. Final
corrections are made with a 3mm hartz burr in cutting the finshed
hole size and seat for the 3mm stone.
The above method works for stones of equal size as well as for
stones that are different sizes. The main thing to undestand is that
the marking-out is a guide, but the drilling and enlarging of the
holes produces the final precision and visual balance.