Calibrating a reflectometer - Scale uncertainty with reading?


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I am calibrating a reflectometer which measures 0/45 reflectance from 100.00% down to 0.00%.
The standard I have is a white plaque (traceable to NRC and NIST by 10nm wavelengths) with about 80% reflectance.
The stated std uncertainty of the original NRC plaque is 0.25%, then my expanded uncertainty ends up being 0.56%.

Now the question: can I scale this uncertainty down so that at 40% reflectance it is half, then 20% reflectance half again?
Otherwise for a 1 or 2% measurement I end up having to state the uncertainty as 0.56% which is to big a part of the chunk!
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