Check out my blog where I compared my HRM, Spark, and the machines, for two magnetic based devices, the elliptical and exercycle. www.sparkpeople.com/mypage_public_journal_
Then check out where I compared whether Spark is (wrongly) using your weight to calculate burn on machines that don't use your body weight, like the elliptical: www.sparkpeople.com/mypage_public_journal_
These will show you how you cannot use Spark's entry. At all. Just forget it.
If you don't have an HRM, then use the machine, or google some other sites for an average and see how they compare to the machine.