All methods have their limitations.
Even heart rate monitors can produce misleading results for those with high blood pressure or some medications.
Online calculators tend to be reasonably accurate for well-studied activities like running, especially as the major factors in intensity (speed and bodyweight) can be fairly objectively measured.
I am surprised that Spark's estimate was so low, unless you were using the mapping based tool, which often gives quite weird calorie results.
In any event, 50 calories here or there is no big deal in nutrition - the body is designed to handle unders and overs like that. It is when your activity level estimate is regularly off by 500 calories that it is more concerning.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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