For running in the flats:
https://fellrnr.com/wiki/Calories_burned_running_and_walking
For mountain runs I look at work reported by Stryd. In the flats Stryd is pretty much aligned with the table above.
I spent quite some time looking into this relationship, e.g running vs calorie consumption. Taking into account all uncertainties I this is best estimate I believe one can come up with. And if you’re not running ultras I’d say the margin of error isn’t that important anyway.
Garmin/Strava or any other HR-based estimate: nonsense.