I’ve been using my Garmin watch for treadmill workouts which is self calibrating to my outside runs. While I am not convinced that it is perfectly accurate wrt pace/stride length, the cadence is be good, and of course the HR is accurate.
I can get incline information for planned races from other sources (I use Ride with GPS) and use that information to include some hill efforts into my treadmill workouts to practice the effort.
I think that is enough even though the mathematics is lacking.