I'd suggest investing in a Heart Rate Monitor! It tells you what your heart rate is during the exercise, and counts how many calories you're burning based on your heart rate, height, weight and gender. I've found that it's a really motivating tool--I got it for Christmas last year from my boyfriend (I have a Polar F4). I think they're about $80. I think that would give you the most accurate calorie count (as opposed to using a generic formula).