You can calculate the drag from a constant altitude flight in 0 wind, if that is what your asking.

In order to maintain a constant altitude you know the vertical component of the thrust must be the weight of the copter, from the lean angle you can then calculate forwards component. At a given lean angle you will reach some steady state speed and hence the forwards thrust component will be equal to drag.

There are too many variables to determine that. I have a small 210 quad that will fly in gale force winds, while a much larger craft is tossed around at 25 kph.