FatCal, Fat % and Carb % are determined by which zones you were in during your workout. Ultimately, the total number of carb calories will depend on your heart rate throughout the workout as related to your VO2 max. In lower heart rate zones, you’ll tend to burn a higher ratio of fat to carb calories, while in higher zones, you will burn more carb-based calories.
Currently Fitdigits only does the calculation on the completion of the activity, it does not support real-time calculations.
You can view equations used at the bottom of ShapeSense’s article.