Treadmill Calibration

GarminSuxLately said:

Also, if you are going to tell me just recalibrate it after my next activity, there is no option as I have already looked

The option appears only once you reached at least 1 mile or 1.5 km on the watch (not on the treadmill). That works for me, and so it is described in the following article, though I saw a post of someone who had to always run a longer distance than at the preceding calibration. I am not sure whether that post is credible, since I only need to surpass 1 mile in order to have the calibration option available, and there is no such requirement in the documentation either. 

How Does the Treadmill Calibration Feature Work on My Garmin Watch? | Garmin Customer Support

Please note, that the distance estimation will vary with the speed and cadence - even if you freshly calibrate, and run faster the next time, or move your arms more intensively, it will result in a greater distance estimation. For that reason I recommend keeping a steady pace at your average level, and swinging the arms naturally, without holding on the railing, when doing the calibration run.