Stryd is accurate out of the box without calibration for most runners. However, there can be small variations between runners and devices.
The Stryd app for the Apple watch does not allow a calibration factor at this time. If you find that your Stryd is not 100% accurate, and you would like to finetune the accuracy, you can use the iSmoothRun app on iOS. iSmoothRun allows setting a calibration factor when used with Stryd.
If you need additional assistance, contact firstname.lastname@example.org