In various papers people have used different areas for calculations of the coefficients One may typically use the projected area. So, you are looking at the wing from the top, the projected area is typically used for getting the lift and drag coefficient. If you compare nondimensional coefficients with other experiments, you always need to make sure that exactly the same reference areas and lengths are used. Eventually it doesn’t matter which area is used as long the results compared are normalized the same way. In the end, it’s important to know what area was used to normalize the data in the report/test data.