All dimensions are in mm.

The inner band is sized to make a size 7 1/2 ring band. The outer band is slightly larger than the inner band so that after the 2 bands are joined it can be trimmed to make a flat surface for soldering the flat outer sheets. After trimming you want the width of the ends of the outer band to be 12.4 mm and the gap to also be 12.4 mm. Since 20 gauge sheet is .8 mm thick this will give you a 14 mm square at the top of your ring.

If you wish to adjust the size of the finished ring change the length of the inner band accordingly and adjust the length of the outer band up or down by the same amount.

Credit where credit is due, these proportions were taken from the box ring lesson in Revere's Book "Professional Goldsmithing".