Many Garmin and Coros watches allow Stryders to set a calibration factor. The Stryd power meter is accurate out of the box and should not require calibration. To measure actual measured distance is not straightforward, but a fun exercise:
If you run a known distance on a track or similar, you can calculate the calibration factor for the Garmin/Coros watch:
(Actual distance / Measured distance) X 100%*
*Actual distance is the distance that you ran (based on an actual measurement or by doing a track test).
*Measured distance is the distance that Stryd measured
On Garmin watches, you can set the calibration factor on the watch by going into Settings --> Sensors and Accessories --> FP-(number). Scroll down to make sure the Garmin always gets speed and distance from Stryd. Scroll down more to the Calibration factor. Enter the number calculated above.
On Coros watches, go to System > Accessories > Added List > Stryd. Check Distance Setting to make sure Coros is getting pace and distance from Stryd. Select Calibration Factor to enter your calculated calibration factor.
Once you have changed the calibration factor on the watch, the Garmin/Coros reported distance will be different than what is recorded by the Stryd pod itself (the calibration factor only affects the watch). If you set up your PowerCenter to import data from Garmin Connect or the Coros app, your distance in PowerCenter will reflect the calibrated Garmin/Coros recorded distance.