I assume you are referring to a Preston FI+Z 3
If so order the Meteric Rings from preston and switch the Preston to metric.
Select Menu, scroll down till "Lens" is highlighted, the midle soft key should then read "ring", select "ring", this takes you to the "select focus ring" screen, the middle soft key will now read "metric". Select it. Now press the left or back side of the 4 way joy stick which is opposite the run switch twice. Your display will now display where it thinks you lens is set in meters, provided that focus mapping is on. You could also use this display to make your own rings as you had previously mentioned.
Good Luck.