Last time I checked, which was just yesterday by chance, my 700 X 23 tire was within 3 mm of the computers published circumference value (2133 vs. 2130) That 3 mm was well within my measuring error, since I used a wide piece of chalk and eyeballed it. So set your circumference value based upon the computers suggested value for your size wheel AND TIRE. But its not that hard to measure your bikes wheel/tire circumference: just put the valve stem at the bottom, make a mark on the tire and ground with chalk, roll the tire along the flat ground one revolution, make another mark, then measure the distance between the chalk marks. As I stated earlier, taking the 2 minutes to do this just verified that the manual recommendation was accurate.

On one bike I once had about a 2% difference, presumably due to the tread pattern being different than typical tires of that size. Being off 2% was no big deal, but neither was taking the 2 minutes to measure. Realize that your computer calculates distance and speed my measuring revolutions of the tire, and with a known circumference, revolutions multiplied by circumference equals distance, and revolutions per time unit multiplied by circumference equals speed.