How to Calibrate Your Treadmill with Your Fitness Watch

I am trying to figure out how to calibrate my treadmill with my fitness watch. I am training indoors in Ottawa during the winter and there is a huge difference between what the treadmill says and what my Garmin shows. Usually the watch thinks I am running much slower than I actually am.

I know there is a calibrate and save feature but it never seems to stay accurate for different paces. If I run a recovery pace it is fine but as soon as I do intervals it goes way off again. It is making it very difficult to track my indoor mileage properly.

Does anyone use a foot pod like the Stryd for better accuracy? I have heard they are the only way to get true pace data on a treadmill. I would love to hear your tips for keeping the data consistent so I do not have to manually edit every single run.