gencalc inquiry

I have a question here, in gencalc http://gencalc.com there is no double factor crest they show just sf, why ?! , is it a lethal gene ? and if so i don't think mating crest to crest will be any better than mating crest to normal, also i believe that the crest mutation is act exactly like the dominant pied, you can get heavy pied from light pied, and vise versa, and there is some opinions about lethal gene, also i would like to know simply from the experts who bred it for long time, what they got from simply mate a bird with full rounded crest to normal bird ?