I've done a good number of FWH only beers and calculating the bitterness as a 20 minute addition would have been flat out wrong.
..and no I didn't do it both ways and do hundreds of blind three way tastings. I don't need too.
I've brewed many FWH beers, and I would have to agree with Malticulous. The bitterness is smoother and more refined than a normal 60 minute addition, but the perceived bitterness is way more than what a 20 minute addition would give you. I'd say it's probably more like a 45 minute addition. Also, I have never sensed any hop flavor or aroma coming from FWH. Just a smoother bitterness. I think the belief that FWH somehow adds hop flavor or aroma is one of the big fallacies of FWH. At least, that's how I perceive it. And you know what they say, perception is reality.