Some bike computers don’t allow you to put in the big numbers for the 36er circumference (who knows why, but I’ve seen posts about this as a problem in the past), but people have solved this by putting a second magnet on the spokes (180 degrees offset for the two magnets) and inputting half the number calculated for the entire wheel. I hope that made sense …
Oh, and if you’re looking to be really accurate, be sure to actually do an on-uni rollout measurement of the tire circumference, as the numbers from a tire company, or even from rolling out the uni without your weight on it, can yield numbers that are off somewhat. But if you’re not anal about this, just a rollout without being on the uni is close enough.