Many Garmin watches allow Stryders to set a calibration factor. Stryd 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 ran a known distance on a track or similar, you can calculate the calibration factor for the Garmin watch:
(Actual distance / Measured distance) X 100%*
*Actual distance is the distance that your wheel measured for the track
*Measured distance is the distance that Stryd measured
You can set the calibration factor on the watch by going into Settings --> Sensors and Accessories --> FP-xxxxx. 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.
Once you have changed the calibration factor on the watch, the Garmin reported distance will be different than what is recorded by Stryd. If you setup your PowerCenter to import data from Garmin Connect, your distance in PowerCenter will reflect the calibrated Garmin recorded distance.
if you need more assistance, contact firstname.lastname@example.org.