but mom, math is hard!
Eh, who needs clickthrough rates. From the Facebook Flyer FAQ, the answer to the question "Can I see how many people clicked on my Flyer?"
Unfortunately we can't provide you with a click-through-rate for your Flyer. These rates vary drastically from one Flyer to the next because they depend on the interest that is generated by the ad's content. The value proposition of Flyers is primarily the high volume and localized exposure of your ad, not click through rates.