@UpDownAround:

I looked into those charts that you referenced in a previous post. It assumes a level grade. There is also the heart bpm that is not considered, which I think is the important element here, besides the grade. Either the calories were pretty close to the chart, or sometimes there was a difference up to at least 20 calories per mile. Some were more, but I am suspicious of them. My watch also underestimates calories burned. I think the heart bpm as a proxy to health caused some of this difference. I discovered the pace effects the result too.

Update:

After further study, I found that both heart rate and pace needs to be high enough to be accurate on my watch with respect to calories burned. Pace effects this more than heart rate. Physical health can have a noticeable difference too. I need to walk at least 3.5 mph with a heart rate at least in the latter 130s for the calorie calculation to be accurate.This is in the range of a good cardio work out which is where I want to be anyways. Looking at the charts now, my calories burned is from what is on the chart to within maybe 10 calories per mile. However, keep in mind that my watch underestimates calories burned.