This is a great question, and we've answered it in a chart in the Cocoas and Dark Chocolates Review. The chart shows the number of calories in popular dark chocolate bars needed to get an equivalent amount of flavanols (200 mg). The results are based on our laboratory tests of these bars -- and they are surprising. They show that, depending on the bar you eat, you may be consuming 3 to 4 times as many calories as you really need to.
Unfortunately, labels typically don't tell you the amount of flavanols in a product, so you can't figure this out on your own. We have found that the % cocoa (or cacao) listed on labels is not a reliable indicator of flavanol concentrations. For example, we found a product claiming "72% cacao" to have more flavanols per gram than one claiming "90% cocoa," and among bars claiming about the same amount of cocoa, one had twice the amount of flavanols as the other.