I set mine by measuring the radius (from hub centre to floor/ground), and subtracted an appropriate amount to compensate for the loaded condition (i.e. me being on the bike).

But there's a much easier way to do it, if your computer allows you to enter your own measurement.
First, set the wheel size as you normally would.
Ride a known distance, and check what the computer says (make this a relatively long distance - maybe 40-50 miles or so).

Let's say you set the wheel circumference to 2105 mm the first time. Your ride happens to be exactly 47.13 miles (according to some digital map/route planning system), and your computer shows 47.92 miles.
Now, divide 47.13 by 47.92, and you get 0.984. Next, multiply 2105 by 0.984, and you get 2071 mm. Enter this new circumference into the computer.

This is, of course, just another form of rollout test, but a much more accurate one, and a more FUN one! You get to ride too!