Convert the spectacle Rx to minus-cylinder form, vertex-compensate both principal meridians to 0 mm, then add half the cylinder to the sphere. Clearlab's keratometry shortcut gives one stock design: for an average K of 39.50 to 41.50 and higher, 8.6 mm base curve / 14.2 mm diameter. Trial-fit and judge lens rotation off the single 6 o'clock line.
