If you have a GPS, you can use it to calibrate as well. Any GPS will do, as long as it can run on batteries and you can somehow transport it on your bike. Borrow a friend's Garmin 705, use the one out of your car, a handheld one, whatever. Doesn't need to be mounted nicely, in a jersey pocket would be fine.

Here's the procedure --

Set up your cyclocomputer. In particularly, you have to know exactly what number you entered into it (the units don't matter.)

Pick a route that is relatively straight (corners may reduce the GPS accuracy slightly) and has no tunnels (thick tree cover is OK if the GPS doesn't mind it), zero the GPS and your cyclocomputer, ride. The longer the better, but a few miles is plenty.

At the end, make exact note of your cyclocomputer mileage reading and your GPS mileage reading.

Once done, take your old cyclocomputer, and plug it into this formula

(new number) = (old number) * (GPS distance) / (cyclocomputer distance)

then reconfigure your cyclocomputer with the new number. Then run the test again (doesn't need to be the same path or distance) -- the distances between the two should be very close, within one percent. If you want you can do the formula again (remember, the (old number) changed -- use the most recent version) and get a tad closer and then test again.

Note that if you configure the cyclocomputer properly using the table they provide and your tire size, it should be within 5% or so (without any corrections on your part.) If you find it to be off more than that, either you got the tire size or configuration wrong or something else is wrong. In particular, if your cyclocomputer reads way too low, the odds are good that the sensor isn't reading every time the tire rotates and you may want to check that. If it reads too high and is wireless (especially if it goes way too high sometimes) it may be picking up interference from something.