You know, these online "calculators" that try to estimate measures of things that we can ACTUALLY directly measure are just that -- estimates. There's a lot they don't account for, no matter how many variables they try to take into effect. Get a power meter and see your real numbers! My riding has changed DRAMATICALLY since starting to use a power tap, and the wired varieties aren't as expensive as they used to be. They help you acheive your training goals much better than just heartrate, speed, and perceived effort. Ditch the website and check out hub-based power meters.