When you calibrate Stryd it does not affect the values sent by Stryd. It sets a multiplication factor to be applied to the numbers coming from Stryd. With a Garmin watch (and no doubt others) you can set a calibration factor. This only affects how the watch displays and records the data, with the adjusted figures from Stryd.
It is no different when calibrating on Zwift. The Stryd (or other footpod) produces its own numbers. If you want an adjusted figure displayed/recorded then you need to create a calibration factor/multiplier.
You do this by tricking the Zwift calibration process so that when Stryd outputs 8 kph Zwift will believe that you are actually running at 10 kph and so on at the different calibration speeds.
You will need to do the maths in advance for your selected calibration speeds and then use another device - sports watch or Stryd app - to show the speed produced by Stryd. Aim for your 80% target speeds on that device when you calibrate. When you’ve finished, Zwift should subsequently use the adjusted speeds instead of the raw Stryd (footpod) speeds.
The outcome of the calibration is stored in a file and can be edited for perfection. Have a look at this post for an example. Click the expand icon to see the full post…
Unfortunately the post is about removing a calibration, but you can see the calibration numbers in the file. These can be edited and saved for subsequent use when you start Zwift. At least it can on PC/Mac. If using a mobile device I think the manual editing is not an option, but you should still be able to trick the calibration.