OK, here's another way. File a bit of scrap sheet to the profile you want and mount it horizontally on the bench behind the lathe as a template. Have a length of rod sticking out from the back of the cross slide so it contacts the template. The rod should be in line with the tip of your turning tool.
Set the pulley in the chuck, such that the tool is touching it half-way along and the rod is in contact with the middle of the template. Then, cutting away from the chuck and using the cross-slide handle to keep the rod in contact with the template, form one half of the crown. Reverse the pulley in the chuck and do the other half, and if necessary use a file to get rid of any tiny ridge left due to slight lack of concentricity between each operation.
Come to think of it, you only really need half of the curve on your template, but it would probably be easier to judge the shape while filing it up if it was full-length.
Andy