Curves has a vested interest in giving you a higher than likely calorie burn. The more you think you're burning there, the more people will sign up. Heck, they advertise it on the basis of burning 500 calories in their 30 minute workout (virtually impossible).
Isn't a large chunk of the workout strength machines? They don't burn much on their own, because you're isolating and focusing on individual muscle or groups, not using your whole body, to do those.
So ... I wouldn't bother. I'd stick with Spark's estimate and see how the numbers translate to results for you. If you do find that you're losing weight faster than the numbers would suggest, perhaps Curves' value is more accurate for you personally.
Deb, in New Zealand