In an OTN (80 wavelengths in full configuration), the input optical power of an amplifier...

To find the nominal multiplexed input power when reducing from 80 to 10 wavelengths, use logarithmic power scaling:

Start with the minimum input power per wavelength (-16 dBm), subtract 10·log(10) to adjust for fewer wavelengths passing, then add 10·log(80) to scale back to the full 80-wavelength configuration.