It's easy to do, simply count the number of poles on sensor first,
Next jack up one wheel(2 if lsd) mark the bottom of the tyre and rotate one complete revolution whilst counting the turns of the bit that drives the speed sensor, may need a helper much easier.
Then multiply this number by the turns per mile of the tyre, easy to get from maufacturer(if not pm me), then multuply the answer with the amount of poles on the sensor, that equals the calibration number, can aslo tweak with sat nav if you get it slightly out.
tabetha