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
You will probably never get it exactly right because you don't know how acratate the odo was that laid out the course.
Also, even if you had it calibrated perfect in the garage, if you use the front brake hard you are skidding the front tire on each turn and loosing a little mileage each time.
That's why they have the auto calibrate feature.
It is there default settting. I haved used this now for 9 enduro's and as you said earlier it is haed to be exact since different not everyone is the same on course milage. It is what has worked for me. S-12 84.4inches and I still use the 83.5.