To determine your ring blank length, find the desired ring size in the far left column, then read across to the appropriate material thickness. Add 0.5 mm – 4mm to the length of the blank. You can always trim off the end of the metal once you have it in the shape and size you need.
I use this chart as a reference. It is better to have a little more metal than not enough. Size your ring down to the correct size after you have bent it to the right shape.