Or an easier way is to place the sensor from your computer on the left side rear horizontal tube of your frame and fix the magnet to your left pedal crank arm so the magnet passes of the sensor, then set your computer to MPH and set wheel circumference to be 2682, now as you read the computer your speed in Mph is cadence / 10 so 8 Mph = 80 cadence 6 Mph = 60 cadence.

Ok, this method isn't 100% accurate at a speed of 8 Mph showing on the computer your real life cadence will be 80.007 which I would accept as close enough.

No more calculations to do