The formula and process:
This is for a 20cm x 30cm diameter lampshade with a height of 25cm
Draw a right angled triangle with a base as 30cm and mark the height at 25cm.
Either using fomex or a long piece of baton, secure using a screw at one end. Measure (in this case) 75cm and 50cm from the secured end and drill a hole big enough to put a pen or pencil through. Make sure that its just the tip that's poking through as you don't want any movement. Draw an arc by placing your pen in the 75cm hole (this will be for your 30cm ring) then draw an arc at 50cm. Then draw a straight line connecting the two arcs. It doesn't really matter where just a straight line somewhere near the middle. Cut a piece of string for the bottom and top arc.Then measure out- in this case- 957mm for the bottom bigger arc and 643mm for the smaller arc using the centre line as your centre reference (you can get these measurements by going to the Needcraft website if you want to use bigger or smaller rings). Then draw your end lines connecting the ends of the arc. Cut out your template.