Whenever I have made templates for drilling discs I have started with a blank circle of cardboard that's oversize for the disc. I then place the disc to be drilled onto the cardboard and draw around the circumferance. The disc can then be set to one side. To find the perfect centre using a drawing compass, bisect from any point on the drawn line, one at say the 12 o/c, the other at say the 6 o/c position, it dosn't really matter. Then transfer the compass to the other side of the template, near enough opposite of the first position, and bisect the lines that have already been drwn from the other side. Then draw a straight line from where the drawn lines bisect each other. Do the same thing for say the 3o/c and the 9o/c positions, draw the line again, and where they cross is the perfect centre. You can keep on bisecting the disc into quarters, eighth,sixteenths,thirtyseconds,etc. Then, using the perfect centre, draw a line with the compass onto the template that is the inner line for the swept area of the disc. Now divide the swept area, say 50mm wide, into 10mm circles onto the swept area of the disc template. You should now have a series of circles on the template which equates to the swept area of the disc which are perfectly divided by the lines bisecting the template into 1/16ths,1/32s,1/64s etc. You can then draw your own chosen pattern from the lines where they cross.Once done, the centre part can be cut out and taped onto the disc to be drilled. Once one disc is done, if a second of a double disc setup is to be drilled, then just flip the template over and do the same again.
Simples.
