So how do I set up Fenix 6 to correctly manage my calorie count?

Garmin uses heart rate and power (if available) for calorie calculations.

The biggest factor in getting more accurate calorie data is ensuring you have an accurate maxHR (enter it into Garmin Connect, rather than the watch).  The age-based default for maxHR is OK for the average of the population as a whole, but rubbish at predicting maxHR for any one individual.

Also make sure that your Activity Class is set correctly in Garmin Connect and make sure your VO2max are up to date for running/cycling if appropriate.

I am 51 years old 73kg 175cm tall I burn about 3000-4000 calories a day according to garmin connect and its synced with myfitnespal and I count my intake of calories and I eat to compensate like 3000-4000 calories a day and I check my weight etc (Garmin scale) and waist almost everyday so I find the calorie burned to be pretty correct and a stable weight. One have to check the activity class. I use the NEAT factor everyday to burn more calories and I can reach my max HR at about 170 outside an activity with my Fenix 5X, I have read that some with Fenix 6 only get maybe 120 HR ??? or similar outside an activity and then the calorie count would be incorrect and useless for me.

Hi. I’m trying to find this in Garmin connect. Will update shortly. 

Have found the setting in Garmin connect device app... it looks like it already had my settings as it must learn from wearing the watch during activities ?

So are we saying the algorithms in my case are correct and the calories count is accurate?