Hello, Can you please provide more information on how to calculate the reflectance of a non-grey standard?

An ideal grey standard is perfectly grey (i.e. has equal reflectance across the entire spectral range the camera is sensitive to). However, sometimes a standard might be dirty, or you need to use a non-grey standard. This option allows you to specify different reflectance levels for the different image channels. You can work out what these levels are by photographing your imperfect/non-grey standard against a true grey standard, and measuring the reflectance of your non-grey standard relative to the true grey.

Non-grey Standard Reflectance
Answered question