Paddling is hard to get right because there is a wide range of calories burned. You can't even base it on speed; I have a 9'6" inflatable board shaped more for surfing than touring. The 4 guys I paddled with last Thursday were on 14' racing hardboards. They had less resistance per stroke, more acceleration per stroke, more glide (less deceleration) between strokes and tracked better. They went faster with less effort. Look at the ranges within each type of SUP paddling and the range across types:

(click on picture if it is too small for you to read)


There is no way an app can get that right for every situation.