I think the assumption has been made that the crested gene is dominant, when it's really incomplete dominant.
Hence my suggestion that you not rule out cresteds unless you'd had the ducks for three generations. If a parent of one of your drakes was a crested, there would be a 50/50 chance of him carrying the gene. I don't work with cresteds so I can't speculate on the chances of him carrying the gene and not displaying the trait, but it doesn't really matter I suppose.
So drake is struttin' around your yard with his incomplete dominant CR gene, mates with a female without the CR and creates a few CR/cr ducks and a few cr/cr ducks. Because of incomplete dominance not all the carriers (CR/cr) in this generation will display, but you would have the same 50/50 split of carriers/non-carriers as you did in grandpas offspring.