Page 6.7 of my copy of the FCTM suggests that the flare should be initiated at 20ft and the thrust levers then retarded, which seems to agree with what has been suggested to you.
It works for me most of the time, although I'm sure there will be those with more experience on it than I have who may think slightly differently, but it was what I was taught when I did base training, and has worked for the three airlines where I have flown it. The NG is slightly different, and I'm sure that the 200 is also, although I haven't flown it. As Sciolistes suggests the speed increments etc should be as boeing suggest, and if you do what they say to it works well.