Ok best way to measure the front tire, on a hard flat surface (garage floor) rotate the tire till the valve stem or rim lock is straight down. Mark that spot with chalk or a marker, rotate tire one time. Mark that spot on floor. Then measure the two marks and enter into the Dog.
Ok on race day, you more than likely you will have to adjust your wheel size to match the computer/bike that miled the event. there will be an odo check at 2.9 miles (in the states anyway, Not sure how they do it up north)
at that point you can adjust your tire size on the go with the watchdog.
Hope this helps