calibrating SRM Powermeter

Any ideas on how to properly calibrate my SRM if I´m not on a smart trainer but rely on the SRM for wattage?

Huge discrepancy between wattage displayed on Zwift and (proper) Garmin Egde.

Here is a link to the manual for the SRM:

Calibration will not resolve this issue, Zwift just picks up what the power meter is broadcasting, just like the Edge. 

Do you have 10 second Avg on the Edge and 1 second on Zwift?

What do you mean by huge?