The judgemental determinability of how "fuckable" someone is, at the discretion of each person. An attractive looking person could have a low fuckable factor(for example, if they had a flat ass). Likewise, a somewhat unappealing looking person could have a high fuckable factor (if, for instance, you didn't have to look at their face).
"While a person may think Rosie O'Donnel is ugly, they might determine that she has a high fuckable factor because she has a big ass."
"Ellen Page may be a bit flat-chested, but she's got a fairly high fuckable factor in other regards."