I'd love to know the answers to your questions, too!
It depends on the videos you are doing. We do some crazy behind the scenes math that I don't even understand (I leave the maths to the science nerds here). However, you can always add custom activities.
Learn how to create custom activities here: http://screencast.com/t/mK7pZXdv
However, you'll get a more accurate calorie count if you use a heart rate monitor or other tracking device.
Does that help?