I also did it using my garmin. I rode till my speedo read 90 mph. GPS read 73. Went on their website and used precise calibration and gave me a -18.9 error. I entered that in my speedohealer now it is dead on at any given speed confirmed with my GPS. I'm 1-/+2 gearing.
I used my Garmin also. The "estimated" calibration will get you close to the actual mph once you input your tires sizes and gearing but if you use a GPS, the "precise" calibration will get you spot on.