Hi,
Use the departure formula for your example, because it is for calculating distance between two point on the same latitude.
You use the convergency formula for calculating change of direction of a great circle track. Not for calculating distance, unless perhaps the average latitude and the convergency is given, in which case you could calculate the change of longitude.
To put it simple, departure and convergency has little to do with each other. One is for distance, the other for degrees.