All you are doing is multiplying diameter by 3.14 (aka "Pie") to get circumference. Basic math. Some computers want centimeters, some want millimeters, others want some goofy formula. Your's just wants millimeters which is not uncommon. A roll out test will give you the most accurate results.

But for future reference, Sheldon Brown has a master-chart of most all wheel sizes.

http://sheldonbrown.com/cyclecomputer-calibration.html

In your case...use column "F"

Don't get too anal about this. Even if you only use the chart instead of a roll out, you still have better accuracy than the speedometer/odometer on most autos. FWIW .... I just use charts myself.