More accurate treadmill distance for Fitness+ workouts

I use a lot of treadmill workouts on Fitness+. I use my apple watch to log and monitor heart rate, distance, and pace. But the distance is always off and shorts me at least 2 tenths of a mile per mile, which adds up.

When I sync my apple watch with the treadmill, it makes me close the F+ workout and start an "indoor run/walk" workout. So my options are boring music with manual workout plans (me setting timers for hard & all out) or Fitness+ workout with inaccurate information.

Is there something I'm missing or is this just a missing feature?