Apple Watch with under-desk treadmill walk

What steps can I take to get the most accurate results on Apple Watch Activity while walking on an under-desk treadmill?

At all times during my walk, my hands are on my desk, and are therefore not swinging. I've tried using the Indoor Walk Activity on the watch, but the distance and calorie count is significantly different than what's reported by the treadmill.

My watch should be properly calibrated as I've had it on me for 3 years and have done plenty of outdoor walks/runs. What else could I do?