# Total retardation of a waveplate in visible light

I have some polymer waveplates with different retardation values over the visible light range. I have the retardation value for each wavelength over the visible light range (in 10nm steps), but I am interested in total retardation of the waveplates when measured over the full visible light range at once. How do I determine what the observed retardation value would be in that case?

For example, if I had the following retardation values:

150nm retardation at 400nm wavelength 138nm retardation at 560nm wavelength 120nm retardation at 720nm wavelength

and wanted to know what the retardation would be over the whole range, could I average the retardation values ((140+138+136)/3)=138nm) and that would be the measured retardation value if one were to measure retardation over the whole 400-720nm light range at once? Or would I need to do a weighted average, as the degree of transmission through an analyzer differ based on retardation and wavelength?